Open Mac Mail client and select Mail from top menu

 

 

Select Preferences ....

Select Accounts

Select Advanced

Towards the bottom of the window check off Use SSL

 

 

 

 

 

When you access your e-mail you will be presented with this window

 

 

Click the Show Certificate button in the error message.

The certificate appears with a certificate icon in the upper-left corner

 

Hold down the Option key and drag the certificate icon to the desktop

Double-click the certificate icon on the desktop, and choose X.509 Anchors from the pop-up menu

 

 

Click Add.

The certificate will be permanently accepted.