Iphone -- sending email issues

For some reason, my phone won't send email - and it seems to be because it can't verify the SMTP server. I am receiving email, and can respond to some of the messages, but not all of them.  My username and password are correct and I'm using SSL with port 587. I've turned the phone off and on, reset network settings, double-checked wifi and LTE settings and I even tried deleting the account and setting it up from scratch - despite all that, it will receive email but won't send.