Every mailbox you connect is tested straight away, and each attempt is written to the mailbox timeline so you can see when a connection started failing.
Common SMTP settings
Port 587 with STARTTLS is the most common configuration. Use an app password rather than the account password — most providers reject the account password for third-party SMTP outright.
When the test fails
Check the host and port first, then confirm the mailbox is allowed to use third-party SMTP at all; several providers disable it by default for new accounts.
Domain verification comes first
A mailbox cannot become active until its sending domain is verified. If the mailbox stays in a pending state after a successful connection test, the domain is the blocker, not the credentials.
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