How to correct server certificate error message

This means that either the site's certificate is not for the full webmail URL, or it has expired. Has it ever worked?

One thing to try is to go to Settings/Passwords & Accounts, delete the mail account, restart the phone (Restart your iPhone, iPad, or iPod touch - Apple Support) then add it back. If the certificate is still invalid (it may not be, as your phone's copy of the certificate would be replaced by this process) you will be prompted to accept the invalid certificate.

You may also not be using the current URL for the mail server. Check the email provider's website.

What is the exact error message? There can be different kinds of certificate errors. Most often they are because the mail administrator let a certificate expire.