A bug in Apple Mail causes affected mail accounts to lose the outgoing mail server (SMTP) information. You can trigger the problem by making changes to the outgoing mail server configuration in Mail –> Preferences. Fortunately, there is usually no need to make changes to the outgoing mail server configuration after setting up an email account. But it’s annoying that Apple has not fixed a bug that I reported in OS X 10.7 Lion. The current macOS version is 10.12 Sierra, and the bug is still present.
How to reproduce the issue
I have four accounts configured in Apple Mail:
- iCloud account
- Exchange 2010 account
- Exchange 2013 (Office 365) account
- IMAP account
Interestingly only two accounts show up in the “Outgoing Mail Server (SMTP)” drop-down list. The outgoing server related to the IMAP account always shows, plus the outgoing server of the selected account.