Your username is 'firstname.lastname@example.org' or 'email@example.com' in the full form.
Standard access (available to both Standard and Premium users):
Consult your Client of choice documentaiton like Thunderbird, Mail.app (macos native) or windows clients.
Example Configuration of Apple Mail to CastleMail.io:
imap.castlemail.io Port 143 Uses STARTTLS
smtp.castlemail.io Port 25 uses STARTTLS
Premium Access (includes access to PGP Encrypted emails, storage etc..:
Full featured suite if your account is enabled:
If your device supports it use Exchange Active Sync (EAS) options otherwise use IMAP & SMTP. The system does not support legacy (old) Exchange MAPI or EWS connections.
This system also supports Outlook and auto configuration on some workstations.