Knowledgebase

Both handle standard mail settings well.

THUNDERBIRD

1. Account Settings > Account Actions > Add Mail Account.
2. Enter name, address and password, then click Configure manually.
3. Incoming: IMAP, mail.yourdomain.com, port 993, SSL/TLS, normal password.
4. Outgoing: SMTP, mail.yourdomain.com, port 465, SSL/TLS, normal password.
5. Username on both: the full email address.
6. Re-test and finish.

Thunderbird is also the easiest way to archive mail from a mailbox you are closing: connect, then drag folders to Local Folders.

APPLE MAIL

1. Mail > Add Account > Other Mail Account.
2. Enter name, address and password. It will fail to find settings automatically.
3. Enter mail.yourdomain.com for both incoming and outgoing, with the full address as username.
4. Once created, Mail > Settings > Accounts > Server Settings. Untick "Automatically manage connection settings" and set the ports explicitly: 993 incoming, 465 outgoing, with authentication set to password.

THE COMMON FAULT ON BOTH

Automatic port management picking an unencrypted port and failing. Set the ports by hand.