Server not verified

this is a common issue i think. If you use the native mail client on iOS, you will need to register a SMTP server for outgoing email. This will require the setup of a server corresponding to your email address. This results in iOS registering a certificate for that server. That will be fine as long as the email domain is valid, when not, as in the case when you no longer use an email address or the domain for that address is not available due to expiry, you will receive a message in iOS stating server not verified. Not resolving this will keep that message alert popping up and it will interfere with your use of the phone. This is a system level alert and not app level so you need to resolve this. The remedy is simple. Go to settings - email and find the corresponding certificate detail in the mail client settings. It will indicate which email it is registered to. Then, delete that mail account. Problem solved. do NOT expect help on this issue from Apple SUpport, I wasted an hour waiting for them to only get a smiley face and apology that this was not an issue with iOS but with 3rd party software which is nit the case. The error is thrown by iOS as the issue is originating in the Apple iOS mail client. Hope this helps.