Messages failed to send, remain stuck in Outbox

Today and one other time recently I used K-9 Mail on my Android phone to send a series of messages through a particular Gmail account. However, I got soon got “Failed to send some messages” notifications. I found the failed messages in K-9 Mail’s Outbox for the Gmail account, where they seemed to be stuck. I checked the Sent folder and the messages were not there, confirming they hadn’t been sent. I reviewed K-9 Mail’s outgoing server settings for this Gmail account and saw nothing amiss. Hitting the Next button in that menu seemed to validate the settings. So I wondered whether there was a problem with K-9 Mail, or a problem with Gmail’s SMTP server.

I re-created the messages in Apple Mail and sent them again. They quickly went from the Outbox to the Sent folder on the Mac. Then the webmail version of the Gmail account also showed they were in the Sent folder, as did K-9 Mail. All of these operate via IMAP, and they worked precisely as expected.

So what could be causing the occasional spells of failure-to-send when using the K-9 Mail app?