Cannot verify server identity non stop pop-ups

Immediately after an update, I started getting pop ups saying cannot verify server identity regarding email accts. I have completed deleted the email accounts in question off the phone and it continues to happen. It will pop up 4-5 times in a row when on cellular data only. It does not happen when I am connected to wi-fi. I am currently unable to retrieve my email when I am not connected to wi-fi. Even if I go to outlook.com via safari when on cellular data it tells me it cannot make a secure connection and will not take me to outlook site. I went to my cell phone company and they said it is an apple issue. Please advise.