Even though the settings aren’t listed on http://wiki.dreamhost.com/POP3_Accounts, DreamHost supports secure POP3, IMAP, SNMP, and webmail access.

• Server: mail.dreamhost.com – use this, because the mailserver’s certificate is issued for this domain name. This works even though DreamHost says to use mail.<yourdomain.com>
• POP3 using SSL/TLS: Port 993
• IMAP using SSL/TLS: Port 995
• SMTP using STARTTLS: Port 587 or 25
• Webmail: https://webmail.dreamhost.com – Make sure you enter “https:”, because DreamHost does not automatically upgrade a http connection to https.
• Username: <username>@<yourdomain.com>

If you want to use STARTTLS for POP3 or IMAP, use the following ports:

• POP3: Port 110
• IMAP: Port 143

I recommend using SSL/TLS when possible. Since a STARTTLS session begins as plaintext, because it just adds one (admittedly minor) point of attack. You have to perform a SSL/TLS handshake anyway, why expose yourself to the risk of a STARTTLS upgrade failure too?