In some cases, iOS Mail (used by the iPhone, iPad and iPod touch) may unexpectedly quit. In many cases, this issue is caused by problematic mail messages stored on IMAP servers.
In most cases, deleting the problematic messages will resolve the issue. Follow this process:
- Open your email account(s) in a desktop computer mail client or a web interface (e.g. gmail.com)
- Delete any messages over 6 MB in size and any messages and any “mail delivery failure” messages
- Delete your email account from your iOS device, then reestablish it