A backup connection

Settings → Backup connection. A retry is right for a mail server that is busy for a minute. It is no help to a customer waiting for a password reset when your mail service's password was changed, its free allowance ran out, or it is down for the afternoon. So you can set up a second way out, and it is tried for the same message, straight away, the moment the main one refuses it.

Choose Another mail service (a second account, ideally with a different service from the main one, so one outage does not take out both) or The web server, which needs nothing set up and is better than nothing. A service chosen from the list fills in its settings, as it does for the main connection, and its password is stored sealed the same way and left out of a settings file.

When the backup sends a message, WordPress is told it went, so a checkout or a contact form carries on normally. The log marks it Sent, with Sent through the backup connection and what the main connection said beside it, and the Dashboard says how many the backup sent this week, because the main connection still needs looking at even though nobody missed a message. When both refuse, the log keeps both reasons in one line, and the message is kept and tried again as usual.

A mail server can also take a message for some of its addresses and refuse it for the others, a stale Cc, say. Email Delivery recognises that, with or without a backup connection: the message is not sent down the backup, nor tried again later, because the addresses that were taken already have it. The log marks it Partly sent (or Refused, when everybody on its To line was refused), says Sent to some of the addresses only and quotes what the server said about the others, and the same is true when it is the backup, or a later attempt from the retry queue, that reaches some of the addresses. Earlier versions needed a backup connection for this; without one, such a message was logged as refused and tried again, and everybody who had it got another copy each time. Send a refused address the message again from its page in the log, once the address is put right.

Send a test tests each connection on its own: Send the test uses only the main connection, so a test is never quietly rescued by the backup, and Test the backup connection uses only the backup.

If the main connection is already the web server, choosing the web server as the backup adds nothing, so it is not tried a second time.

It works by using a copy of WordPress's own mailer that can fall back, in place of the plain one, so nothing else about how your site sends changes. The copy is in place with or without a backup connection, since it is also what tells a message some of its addresses have. If another plugin has already replaced WordPress's mailer with its own, that one is left alone: the backup is not tried, and a message the mail server took for some of its addresses is logged as refused and tried again.