Using Outlook for iOS in business environments for email and scheduling
I have the very same problem - and no, i do NOT have more folder that could confuse the phone! I only have "Sent".
Also i have checked the webmail and the messages that i sent are not to be found anywhere in any folder.
To add to the confusion: I *did* work in the beginning! It just stopped working...
I'm sure that when i delete the account on my phone and then reinstall is, it will start working again (like it did before after re-installing) but that we'll see the problem re-appear again.
I will avoid doing that again as then it will start synching from the Sent folder on the IMAP server, and, again, that one misses the emails that i sent from my phone. It only contains the ones i have sent using my laptop.
It all seems there is a hidden 'feature' like "Only keep Sent emails on my phone, don't synch with IMAP server"
I'm using a hosting company's mail servers, this means i'm loging in as "******@mydomain.com" at a mailserver which is "IMAP.hostnameprovidersname.com" using the 993 port etc.