We previously noted an issue in which mail sent from the iPhone simply disappears into the void, never reaching its recipient, nor appearing in the "Sent Items" folder, and also never generating an error message indicating that anything has gone wrong.
In many cases, the fix for these issues is the same as one we noted for other mail-related problems: delete and re-establish the afflicted account. Tap Settings and then tap Mail. Tap on the problematic account, then scroll to the bottom and select Delete Account. Go back to the main Mail settings pane and tap Add Account to re-create the account, re-entering your settings. Note that you?ll lose any mail downloaded to the iPhone from non-mirrored POP accounts.
If you are using a Yahoo! or Gmail account and experiencing these problems, you may need to re-establish the accounts without using the predefined settings offered on the iPhone. Instead set up the accounts manually.