Both incoming and outgoing servers must be set to use an encrypted connection.
If your client is not listed, use the settings below:
Name: Your Full Name
User ID or Account: Your RCSID
Incoming mail server: mail.rpi.edu
IMAP: port 143 with STARTTLS (preferred) or port 993 with SSL/TLS
Outgoing mail server: mail.rpi.edu
SMTP: port 587 with STARTTLS (preferred), or port 465 with SSL/TLS
Outgoing server requires authentication: use port 587 with STARTTLS for encryption (sometimes called TLS or SSL for historic reasons, or "encryption"). This is preferred, but if that does not work use port 465, which is an SSL/TLS encrypted connection.