Without knowing more about your mail stats and settings, it’s hard to guess what triggered this email to be sent.
However… Dreamhost really shouldn’t do these 48 hours or die emails. If there is a problem with your email account size, that problem certainly wasn’t created in 48 hours. It doesn’t surprise me that dreamhost sent an email like that, they’ve done similar things over the years. 48 hours isn’t enough time to deal with it (what if that email came while you/I were out of the country?) On the flip side, I also know users tend ignore things that say 30 day warning.
If I were able to advise dreamhost how to handle such a situation, I would suggest the policy first become more black and white, Second, the sent emails sent should link the policy and be explicit about which part of the policy is causing the problem. Finally, the emails for something like this issue should have a 30day countdown clock attached and a new reminder email sent each morning with the subject line “Email deletion in [30/29/28/27/26…] days.” (maybe 30 days is long, but 48 hours is WAY too short)
As for the AUP linked above, that has apparently been updated since I last read it. How would I know if I was doing something wrong? You’re also correct that it’s pretty grey about what is acceptable.
Edit to add: Another process that could certainly work is to keep the 48 hour message to get the users attention, but also have a single use automated time extender link. For example a link in the email that says “Need more time? click here and we will extend the deadline 14 days.” When clicking the link the user should be given a specific date and time that the future action will take place.