Read this support article about...
We make use of 'autodiscover' technology, so that in the majority of cases, you won't need to enter any server details into popular email applications.
If automatic setup failed, you can choose Manual setup or additional server types to enter your settings manually. Use these settings:
Incoming mail server:
imap.stackmail.com
Outgoing mail server:
smtp.stackmail.com
Account type: IMAP
Incoming server port (IMAP): 993
Outgoing server port (SMTP): 465
Use the following type of encrypted connection: SSL/TLS
If you experience any problems with the connection or receive a certificate warning, please disable SSL/TLS encryption and change the incoming server port to 143.