Nope. It should be the primary e-mail address for the mailbox as shown in the Web Panel.
Its more complicated than that. The domain should correspond to an IP address, and the machine listening to that IP address should have an SMTP mail server listening on port 25 or port 587, and an IMAP server listening on port 143.
If pings timeout, contact support with a trace route from your local computer to the IP address.
For IMAP you can telnet to port 143 and you should get a greeting from Courier-IMAP. Type GOODBYE and it ends the session and the connection is dropped.
For SMTP (outgoing mail) You can also telnet to the machine on port 25 or 587 - if the SMTP mail server is listening, you should get a "220 hostname ESMTP" line. Type in BYE and then you should get a "221 Bye" line and the session ends and connection is dropped.
For outgoing, Leave SPA turned off, use the primary email address and the "mail" hostname, and try changing the outgoing port to 587.
The webmail hostnames do not correspond to a machine with an IMAP server or SMTP server. The machines are running a web server (hence "webmail") and require a web browser for port 80 or SSL.
To determine if the problem is with your computer / ISP visit https://webmail.dreamhost.com/ and try to login there. If you can't login, contact support.
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