This issue affects only customers who are using "mail.domain" as their incoming and outgoing mail server in the settings on the iPhone. For example, if your domain name is "mydomain.com" you have set both your incoming and outgoing mail server hostnames in your iPhone settings to mail.mydomain.com (you'd replace mydomain.com with your own domain name).
The server's security certificate for Email is tied into the server name, not your domain, so you will always recieve a mismatch when using the domain unless you have your own SSL certificate. So when you originally added the Email account to your iPhone, you will have received the same error you are receiving now - and to stop the error, you will have tapped a button to trust the certificate. The only difference now is that the option to trust the certificate is no longer available in iOS10 and above.
The answer to this issue is simply to change your incoming and outgoing mail server in your Email settings to the server your hosting plan resides on. Most customers already know their server name and it is sent out to you in your hosting details Email, but if you don't know it, you can resend this information...
1. Login to your account.
2. Click My Services > Manage Services
3. Click on your hosting plan
4. On the left menu, click Resend Service Details
This will Email you your hosting welcome Email which contains the correct server names for your incoming and outgoing settings. Simply update your email account in your iPhone with these details and the error will go away.
We've also heard that some customers are having this same problem on Apple Mac computers. You should be able to simply "trust" the certificate again, otherwise you'd need to follow the same instructions above and edit your incoming and outgoing mail servers in your mail app settings.
If you have any questions regarding this issue, please do not hesitate to get in touch.
Friday, December 22, 2017