Mail dropped instead of sent because corrupt account settings
I noticed that emails I sent from one of my accounts wasn't received by the other part. When troubleshooting(1) the issue I found that I couldn't create new emails using this account (all the other emails I've tried to send was replies). Looking further I found that the send-information in the account settings page was gone for this account and there was no way for me to enter it (the "Outgoing mail server" section was missing).
The major issue is that emails were silently dropped instead of sent or a warning/error (or just keeping it in outbox, where I could have found it).
Secondly, how did the "Outgoing mail server" section disappear?
1) I found out that the mail wasn't even placed in "sent", they just disappeared