I am also having this issue.
I was set up with the sub4.mail.dreamhost.com for incoming/ outgoing and had used ports from the wiki, then yesterday I tried to add another account to mac mail and that brought everything down.
I have tried deleting and re-adding the accounts, but it seems that I only get one good try a day and then nothing works.
I have tried restarting and shutting down between tries. Right now I really just want my main e-mail account to be back on mac mail and I’ll worry about adding the second account later.
When I try setting up the account again I get this error:
"The server returned the error: Connections to the server “sub4.mail.dreamhost.com” on the default ports timed out.’
The second e-mail account is on the same server, do I need to set them both to different ports. Are they disrupting each other?