Navigate to Account Settings > Email Settings. 

Select "Other" from the dropdown and enter these settings, along with your email account's username/password:

Server name:  yourdomainname.com (Replace 'yourdomainname' with your custom domain)
Port: 993
SSL: Yes

Select "Other" from the dropdown and enter these settings, along with your email account's username/password:

Server name:  yourdomainname.com (Replace 'yourdomainname' with your custom domain)
Port: 465  
SSL: Yes