Mac/iPhone/iPad imap mail not saving sent message
Sometime after an upgrade, the Mail settings get reset to save Sent messages on the iPhone/iPad.
Here is what to do in order to get your sent messages on the server.
For each email account:
On iPad
Settings>Mail, Contacts, Calendars>Accounts>Choose your IMAP account>Advanced>Mailbox Behaviors>On the Server>Sent Messages.
On iPhone
Settings>Mail, Contacts, Calendars>Accounts>Choose your IMAP account>Advanced>Mailbox Behaviors>On the Server>Sent Messages. (Basically the same thing)