Navigate to Account Settings > Email Settings.
Incoming
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
Outgoing
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