If you are using #k9mail and suffer from endless "Warning: Could not send mails,
Max. Retries exceeded" (or the german equivalent)
it might help to move your mails from "Outgoing" or "Postausgang" to "Drafts" or "Entwürfe" and send them again.
This is a bug at least in version 10.0 but I found this work-around in Internet for versions 9.x !
Of course there might be other reasons for these messages, i. e. wrong smtp server address/password or smtp server down. But in my case this worked.
I killed #k9mail , delete the cache but to no avail. I think that the outgoing mail has a certain state which is stored as meta-data for the mail itself. This assumption is based on an excerpt from the source code found during my investigations.