Provider guide · Postmark
Postmark is the specialist: transactional email only, with deliverability as the whole product. Three things shape the setup: one token pasted into both credential fields, a sender confirmed in advance, and a human approval of the account before it may write to strangers. All three are quick; only the last one waits.
1 · Credentials
Postmark organises sending into servers, and each server carries API tokens. That token is the whole credential story.
In Postmark, open your server, then its API Tokens tab.
It acts as both the SMTP username and the SMTP password.
Under Sender Signatures, add the address you will send from and click the confirmation email, or verify your whole domain with the DKIM record Postmark gives you.
Until a human at Postmark approves the account, it only sends to domains you verified yourself. Once your test works, request approval from the dashboard; weekday answers typically come within a day.
The token is sensitive by nature, since it can send mail as your server. If it ever leaks, rotate it from the same tab: a server can hold several tokens, so you can add the new one to WordPress before deleting the old.
2 · In the plugin
Selecting the Postmark preset fills the server (smtp.postmarkapp.com), the port (587) and the encryption (TLS). What remains is yours.
The Server API Token.
The very same token, pasted again.
An address matching a confirmed Sender Signature, or any address on a domain you verified.
Save, then use Send Test Email. Messages go through the server’s transactional message stream by default, which is exactly what WordPress traffic is.
3 · Pitfalls
Postmark’s strictness is the product working as designed; it just needs to be known in advance.
Until a human at Postmark approves your account, you can only send to recipients on domains you have added and verified yourself; anyone else is out of reach. Approval is requested from the dashboard and typically answered within a day on weekdays. Plan for it: configure, test against your own addresses, request approval, then launch.
Postmark only sends from addresses you confirmed beforehand, one by one as Sender Signatures or wholesale by verifying the domain. A From outside that list is refused. If your site sends from several addresses, shop@ and no-reply@ say, verify the domain once rather than confirming each signature.
Postmark separates transactional and broadcast traffic, and the SMTP endpoint the preset points at is for the transactional kind: receipts, resets, notifications. Newsletters and campaigns belong in their broadcast stream, with its own endpoint. Sending a mailing list through the transactional door is the fastest way to a conversation with their compliance team.
The email log keeps the exact error message the server returned, which is the fastest route to the cause. The documentation walks through the common ones.
There is a guide like this one for each of the twelve presets, from Gmail to Amazon SES, and the plugin takes any other SMTP server too.