Just went to check some emails I sent from my iPhone but there is no sent folder?
If i go to setting > mail >[account] > advanced the mail 'sent' folder is selected but there isn't one in Mail's mailboxes page.
The emails are being sent. They are also not appearing in my sent folders in OS X Mail.
This is the same for 2 imap accounts that i've been using on my iOS devices for 5 years+.
However if i send an email from mail app using my Apple ID it appears in its sent folder shortly after in Mail iOS, and then appears in my sent folder os x mail.