I created a ticket for this (#518135) but have to think this problem is more widespread. I have started seeing extremely slow inbound mail delivery times starting late last week with horrible response times today.
As an example, I sent a test mail from Yahoo! Mail and Hotmail to firstname.lastname@example.org, which is a POP3 mailbox on DreamHost. As of 90 minutes and counting it has not delivered.
When I lookup the MX records for my domain I get:
Doing a TELNET to port 25 on these results in slow or timed out connections.
My outbound mail is fine. I.e. the SMTP servers that mail.mydomain.com points to are OK.