Blog
Practical notes on sending email. The stuff that breaks, the distinctions that matter, and how to let an agent send without regretting it.
- AI Agents
10DLC rules for AI-generated text messages
AI-written texts are still A2P traffic: register the campaign, keep the agent's texts matching its samples, stop drift into marketing, re-register on change.
Unitpost · November 2, 2026
- AI Agents
How to make an AI agent ask before it sends email
Which agent sends need a person's approval, client confirmation vs server-side controls, and a draft, preview, confirm flow with timeouts and an audit trail.
Unitpost · November 2, 2026
- AI Agents
How should an AI agent handle bounces and complaints?
What happens after a bounce or spam complaint, what an AI agent sees on the next send, and rules that stop it retrying or working around the suppression list.
Unitpost · November 2, 2026
- AI Agents
Have an AI agent send you a weekly deliverability report
Read Unitpost sending stats and domain DNS health over MCP or the CLI, have a model summarize bounces, complaints and DNS issues, and email the team weekly.
Unitpost · November 2, 2026
- AI Agents
How should an AI agent handle email API rate limits?
What a 429 from an email API means for an AI agent, how to read Retry-After and the RateLimit headers, and how to retry without sending anything twice.
Unitpost · November 2, 2026
- AI Agents
How to have an AI agent reply to inbound email on your own domain
Point MX at a subdomain, verify the signed email.received webhook, dedupe, then send one threaded reply with an idempotency key from the inbound id.
Unitpost · November 2, 2026
- AI Agents
How to send appointment reminders by SMS from an AI agent
Let an AI agent text appointment reminders: consent on record, a registered use case, a fixed template, the local hour, and one idempotency key per reminder.
Unitpost · November 2, 2026
- AI Agents
How to handle STOP and HELP when an AI agent sends texts
Keep STOP and HELP away from the model: handle keywords first, send one confirmation, honor plain-language opt-outs, log consent, check before every send.
Unitpost · November 2, 2026
- AI Agents
How to let an AI agent answer text messages
Let an AI agent reply to texts safely: the sms.received webhook, messages it must not answer, opt-out words it shouldn't guess at, and when to hand off.
Unitpost · November 2, 2026
- AI Agents
Does AI-written marketing email still need consent?
AI-written marketing email still needs consent. The rules in the US, EU, UK and Canada, Gmail and Yahoo bulk sender rules, and where AI agents go wrong.
Unitpost · November 2, 2026
- AI Agents
Which API key permissions does an AI agent need?
The smallest API key for each AI agent job, from sending receipts to running campaigns, what no key can ever do, and the 403 a missing permission returns.
Unitpost · November 2, 2026
- Deliverability
Gmail, Yahoo and Microsoft bulk sender requirements
What Gmail, Yahoo and Outlook.com require from bulk senders: SPF, DKIM, aligned DMARC, one-click unsubscribe and a spam rate under 0.3%.
Unitpost · November 1, 2026
- AI Agents
Can an AI agent have its own email address?
Three ways to give an AI agent an email address: a person's mailbox, an inbox-per-agent product, or your own domain with inbound routing and a send API.
Unitpost · November 2, 2026
- AI Agents
Get an email or a text when Claude Code finishes a task
Claude Code Stop and Notification hooks that run the unitpost CLI: an email when Claude finishes, a text when it waits for your approval.
Unitpost · November 2, 2026
- Concepts
Why domain verification should never be a one-time check
DNS changes after you verify a domain. How continuous re-verification with a grace period catches regressions without turning a transient blip into an outage.
Unitpost · July 27, 2026
- Troubleshooting
Why DKIM verification fails (and how to fix each cause)
Why a DKIM check fails after you've published the key: a selector mismatch, a truncated public key, DNS flattening, or a message changed in transit.
Unitpost · July 27, 2026
- Deliverability
Email bounce codes explained: hard, soft and policy rejections
What 421 4.7.28, 550 5.7.1, 550 5.1.1, 452 4.2.2 and 550 5.7.26 mean, and whether to retry, slow down, fix authentication or remove the address.
Unitpost · November 1, 2026
- Deliverability
Email copy that hurts deliverability: spam words, subject lines, links and images
What in your copy affects spam filtering: misleading subjects and From names, link shorteners, image-only mail, no text part. Spam words matter less.
Unitpost · November 1, 2026
- Deliverability
Why your transactional email goes to spam
A diagnostic order of operations for transactional mail landing in spam: authentication, reputation, content, and the engagement signals you can't see.
Unitpost · July 27, 2026
- Deliverability
Email list hygiene: how to clean an old or imported list
Which addresses to remove and when, how to send safely to an old or imported list, and where re-engagement, role addresses and double opt-in fit.
Unitpost · November 1, 2026
- AI Agents
What is an email MCP server?
How Model Context Protocol exposes email sending as agent tools, what a well-designed email MCP surface looks like, and how authentication and scopes work.
Unitpost · July 27, 2026
- Deliverability
How often to send marketing email
Choosing a marketing email cadence that doesn't drive spam reports: match what people signed up for, stay steady, avoid double sends, respect time zones.
Unitpost · November 1, 2026
- AI Agents
30 things to ask an email and SMS MCP server
Thirty prompts for Claude, Cursor, ChatGPT or any MCP client connected to Unitpost, grouped by job, with the tools each calls and which ones send.
Unitpost · November 2, 2026
- AI Agents
Write emails that AI inbox summaries get right
What Gmail's Gemini and Apple Intelligence do with your email, per their own help pages, and how to write mail that their summaries get right.
Unitpost · November 2, 2026
- AI Agents
Gmail sending limits and API scopes for AI agents
What an AI agent sending through Gmail runs into: daily sending limits, Gmail API quota units, and restricted scopes that need a yearly security review.
Unitpost · November 2, 2026
- Concepts
How email webhooks work (they're POST)
An email webhook is an HTTP POST to your URL when a message is delivered, bounced, opened, or clicked — not polling, not IMAP.
Unitpost · August 27, 2026
- AI Agents
OAuth or API key: how to connect an AI agent to an MCP server
What the MCP spec says about OAuth, how Unitpost's hosted server signs a person in, what consent grants, and when a scoped API key fits better.
Unitpost · November 2, 2026
- AI Agents
MCP server, SDK or REST API: how should an AI agent send email?
The same email send over MCP, an SDK and REST: when each fits, OAuth vs API keys, and why scopes, limits and errors match on all three.
Unitpost · November 2, 2026
- Concepts
Move off SendGrid SMTP after the free plan ended
SendGrid retired its free plan. Point the same SMTP client at Unitpost — host, username unitpost, API key as password — without rewriting your app.
Unitpost · August 27, 2026
- AI Agents
Send email and SMS from an n8n AI agent with MCP
Connect an n8n AI Agent to Unitpost with the MCP Client Tool and a Bearer API key, or call the REST API from an HTTP Request tool.
Unitpost · November 2, 2026
- AI Agents
Personalize onboarding emails with an LLM without losing control
Keep the onboarding template fixed and reviewed, let an LLM fill named variables from product data, check each value, and send once with an idempotency key.
Unitpost · November 2, 2026
- AI Agents
Send a product update email with Claude and an MCP server
Turn a changelog into a product update with Claude and Unitpost's MCP tools: segment, topic, template, validation and a test send, then a person sends it.
Unitpost · November 2, 2026
- AI Agents
How to protect an AI email agent from prompt injection
Keep inbound mail from steering an AI agent: treat bodies as data, check DMARC and the webhook signature, scope the key, and approve the unusual.
Unitpost · November 2, 2026
- Deliverability
How to recover a damaged sender reputation
What to do when Gmail or Outlook start throttling or rejecting your mail: stop bulk sends, fix the cause, read Postmaster Tools and SNDS, then ramp back up.
Unitpost · November 1, 2026
- AI Agents
How to schedule email and texts from an AI agent
Let an AI agent schedule sends without timezone bugs: scheduled_at with an offset, the 90-day horizon, cancelling, batches and quiet hours for texts.
Unitpost · November 2, 2026
- AI Agents
How to let an AI agent send email safely
The concrete controls that make agent-initiated email safe: scoped credentials, a verified-domain gate, idempotency, approval before send, and an audit trail.
Unitpost · July 27, 2026
- Deliverability
SMS deliverability: registration, consent, quiet hours and carrier filtering
Why US business texts get blocked and how to avoid it: 10DLC and toll-free registration, consent and STOP handling, quiet hours, and content carriers filter.
Unitpost · November 1, 2026
- Troubleshooting
How to fix an SPF record that isn't found
Why an SPF lookup returns nothing even after you've added the record, and how to diagnose it: propagation, wrong host, quoting, and the one-SPF-record rule.
Unitpost · July 27, 2026
- AI Agents
Test an AI agent's email and SMS before real people get them
Test agent email and SMS on inboxes and phones you own: render and test-send templates, validate campaigns, fire webhook tests, use simulator numbers.
Unitpost · November 2, 2026
- Concepts
Transactional vs marketing email: what's the difference?
The practical difference between transactional and marketing email (consent, unsubscribe rules, reputation risk) and when one tool for both is right.
Unitpost · July 27, 2026
- Deliverability
How to warm up a new sending domain
Why Gmail throttles mail from a new domain, what mailbox providers actually publish about ramping volume, and a conservative ramp you can follow.
Unitpost · November 1, 2026
- Concepts
What is an idempotency key, and when do you need one?
How idempotency keys prevent duplicate sends when a request is retried, how to choose one, and the failure modes they don't cover.
Unitpost · July 27, 2026
Free tools
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Roundups
Ranked for one job each, with the weights and reasons in the open.
- Best SendGrid alternatives in 2026SendGrid alternatives ranked for teams leaving after the free plan ended: free plans, API, transactional plus marketing, and how hard the switch is.
- Best free email APIs in 2026Free email APIs ranked by what the free plan really gives you: monthly volume, daily caps, domains, inbound and marketing, and the price of the first paid step.
- Best email APIs for AI agents in 2026Email APIs ranked for AI agents: official MCP servers, send safety, reading replies, agent-readable docs and the free plan. With the reasons for every score.
- Best SMS APIs in 2026SMS APIs ranked for developers by real US price with carrier fees, registration and consent help, API, reach, email in the same product and AI agents.
- Best SMS APIs for AI agents in 2026SMS APIs ranked for AI agents: official MCP servers, consent and STOP handling, API, email in the same product and US price. With a reason for every score.
- Best Twilio alternatives in 2026Twilio alternatives for SMS ranked by real US price with carrier fees, registration and consent help, API, reach, email in the same product and AI agents.
Migration guides
What imports, what you rebuild, and the order to switch in.
Comparisons
How Unitpost compares with other email and SMS providers, including where they are the better choice.
- Unitpost vs ResendA developer-first transactional email API, known for the cleanest DX in the category and for React Email.
- Unitpost vs SendGridThe Twilio-owned incumbent, processing billions of emails a month across transactional and marketing with deep enterprise tooling.
- Unitpost vs PostmarkA transactional-only specialist with the strongest deliverability and time-to-inbox reputation in the category.
- Unitpost vs MailgunSinch-owned email infrastructure aimed at developers, with strong validation and deliverability tooling.
- Unitpost vs BrevoAn all-in-one marketing platform (formerly Sendinblue) covering email, SMS, chat, and CRM, with a transactional API alongside.
- Unitpost vs LoopsA modern, design-led email platform aimed at SaaS companies, focused on marketing sequences and lifecycle email.
- Unitpost vs MailchimpIntuit's email marketing platform, with a drag-and-drop editor, automation flows, audiences and landing pages, and transactional email as a paid add-on (formerly Mandrill).
- Unitpost vs Amazon SESAWS's email sending and receiving service, used through the SES API, SMTP or an AWS SDK, with events published to other AWS services.
- Unitpost vs MailerSendA transactional email, SMS and WhatsApp service from the team behind MailerLite, with newsletters left to MailerLite.
- Unitpost vs AgentMailAn email API for AI agents: each agent gets its own inbox, created by API, for two-way threaded conversations rather than one-way notifications.
- Unitpost vs TwilioA programmable communications platform with APIs for SMS, MMS, RCS, WhatsApp, voice, video and email.
- Unitpost vs TelnyxA licensed carrier with its own network, selling voice, SIP, SMS, MMS, RCS, WhatsApp, email, numbers, verification and IoT connectivity.
- Unitpost vs BirdA messaging platform, called MessageBird until 2024, with APIs for SMS, email, WhatsApp, Apple Messages and voice, plus verification and a marketing suite.
- Unitpost vs VonageEricsson's communications API business, with a Messages API for SMS, MMS, RCS, WhatsApp, Messenger and Viber, plus voice, video and verification.
- Unitpost vs SinchA communications API provider with SMS, MMS, RCS, WhatsApp, voice, video and verification, which also owns the Mailgun and Mailjet email services.
- Unitpost vs PlivoA communications platform, now led by voice AI agents, with APIs for voice, SIP, SMS, MMS, RCS, WhatsApp and verification.
Send email your users actually receive
5,000 emails a month free — API, SMTP, templates, tracking. No credit card.