mail not sent without user notification after change of credentials on server

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Description
After changing the credentials on the mail server the user is notified by K9 for the inbox (wrong password message) BUT not for the outgoing mail! This results in mail being kept in the OUTBOX as unsent. I only recognized that several days after the change of the credentials that emails are stuck in the outbox.

Steps to reproduce

1. Change password on mail server - in my case this is GMX. GMX requires credentials for send (SMTP) and sync via IMAP to the device
2. Compose new email and press send
3. Check contents of OUTBOX folder on the device - the email just created and sent sits there idling

Expected behavior
Error message/warning that the mail could not be sent (possibly with reason).

Environment (please complete the following information):

• K-9 Mail version: 5.600
• Android version: 9
• Device: Xiaomi Redmi Note 4 (mido) /w LineageOS
• Account type: IMAP

Additional context
This is a very irritating behavior: the user is notified about the wrong credentials immediately, as soon as K9 is trying to fetch new mail via IMAP. The correct password supplied in the K9 prompt is only used for sync of inbox and folders with imap.gmx.net (required credentials are user and password) but not for the OUTBOX folder which is connected to mail.gmx.net (SMPT server for outgoing mail; required credentials are again user and password).

Start by reproducing the credential-change scenario with the IMAP inbox and SMTP outgoing server described in the issue, then trace the outgoing-mail send and error-handling entry points. Done means a failed SMTP authentication produces a visible warning or error and the message is not left silently in the OUTBOX; verify the behavior on Android.