The big picture conundrum is that dreamhost offers low cost shared web hosting which in addition to legitimate users also attracts spammers. The current mail issues are slightly different than they have been in recent years. Currently, nobody likes Atmail and sub-4 weirdness got worse.
Sub-4 issues have popped up in threads over the last few years to the point that I've never understood why measures like replacing it haven't been undertaken... lately the sub-4 issues seem different but that only a general sense because I haven't been tracking it specifically but I've read every post in the forum since 2008.
Before the current issues there have always been issue of servers blacklisted. Dreamhost got serious about that and lately it doesn't seem like there are as many blacklist complaints. Blacklist complaints were also an issue just because of the wording of the email that a user would get back from support when they forwarded blacklist information. The wording was basically thanks, you won't hear back anymore about this, there is no way to ask us for an update, we don't know when it will be un-blacklisted etc. It was not very customer service friendly, and some would complain in this forum about that.
Some years ago (maybe 3-5) there was a post in this forum that I can't find anymore where a Green Named employee that had been around dreamhost forever admitted email service was less than it should be, and that they were addressing it. 3-5 years later, nothing much changed at least from the POV we have from the outside looking in.
There has always been weird issues of forwarding and sending email to certain domains... including gmail.
Delays and outbound mail that was just purged from backed up mail queues are the reasons why I finally pointed my MX records elsewhere. That was 3 years ago, and I'm still very happy with the results.
At some point I suggested that dreamhost should offer tiered email. The shared service currently offered for the current price, and a premium business class email on all different servers, domains and IPs, etc. for an extra per user fee. Then those that wanted better email could pay for it, and those that didn't care and wanted to stay on the shared servers could.
After reading a recent post in a different thread, I've wondered if dreamhost could offer a pre-rolled roll your own mail server image as part of it's dreamcompute offerings. Probably not an option for everyone because dreamhost really couldn't provide support it or maintain it beyond the initial config.
As you can see, I don't think dreamhosts email problems are new. In fact they pre-date Simon Anderson. I'll concede that recent complaints have a new melody, but it's really the same song that's been playing like a broken record for years, not months.