Sylpheed IMAP setup for icloud.com mail and Gmail accounts

NOTE (Feb 11, 2022): I don't use Sylpheed anymore and this is likely out-of-date.

It is necessary to go to Apple's Apple ID site and set up two factor authentication, and set up an application specific password for Sylpheed

In Sylpheed "Account preferences" menu:

Basic: Server Information: Protocol: IMAP4
Server for receiving: imap.mail.me.com
SMTP server (send): smtp.mail.me.com
User ID: username (not username@icloud.com)
Password: Use the account-specific password you set up on Apple ID site
Receive: IMAP4: Authentication method: LOGIN
Send: Authentication: Check "SMTP Authentication (SMTP AUTH)"
Authentication method: LOGIN
User ID: username@icloud.com
Password: Use the account-specific password you set up on Apple ID site
SSL: IMAP4: Use SSL for IMAP4 connection
Send(SMTP): Use STARTTLS command to start SSL session
Advanced: Check "Specify SMTP port": 587

It is necessary to go into Gmail, activate IMAP, and go to "Allow less secure apps" and choose 'Allow'.

In Sylpheed "Account preferences" menu:

Basic: Server Information: Protocol: IMAP4
Server for receiving: imap.gmail.com
SMTP server (send): smtp.gmail.com
User ID: username@gmail.com or username@domainname.com if you have Gmail using your own domain name.
Password: Use your Gmail password
Receive: IMAP4: Authentication method: Automatic
Send: Authentication: Check "SMTP Authentication (SMTP AUTH)"
Authentication method: Automatic
User ID: leave blank
Password: leave blank
SSL: IMAP4: Use SSL for IMAP4 connection
Send(SMTP):Use SSL for IMAP4 connection
Advanced: Use default SMTP and IMAP4 ports (654 and 993)