Marcello Apple ... Try one of these ... one should one and there are three (3) to try. If none work, maybe you have a virus ... Wish you luck:
SOLUTION #1: EASIEST IF IT WORKS
no need to reset the iphone .. just close & restart the mail app (while in airplane mode) .. and it works .. thanks all! PS: Take it out of airplane mode when finished.
----------------------------------------------------
SOLUTION #2 WHICH DID NOT WORK FOR ME (BUT IT IS QUITE SIMILAR TO #2)
1) Put the iPhone in Airplane mode
2) Open the Mail app
3) Go to the Outbox
4) Click Edit, click the circle next to the stuck message to select it
5) Click Delete
6) Take the iPhone out of Airplane mode
7) Return to the Mail app. :-)
----------------------------------------------------
SOLUTION #3 WHICH WORKED FOR ME!!!
1) Put your iPhone/iPad in airplane mode
2) Fully switch-off your phone
3) Turn it back on
4) When prompted enter your access code, but don't unlock your SIM
5) Your iPhone/iPad is booted
6) Go to your mail app
7) Go to Outbox
8) You should be able to click on edit in the top-right corner
9) Click on the circle in front of your e-mail
10) Delete
---only for a PHONE ---
You can reactivate cellular and wifi data at this point, turning off airplane mode should prompt you to enter your sim.