I've never had this issue with a personal custom domain. Basically all businesses have custom domains so blacklisting all of them can't make sense, surely?
Using a custom domain doesn't necessarily mean using a custom mail server, they can point to a really common mail server such as gmail for instance. This is how a lot of business emails are set up.
True, but then I would assume that serious monitoring apparatus would include mail server monitoring, therefore defeating the need for a custom domain, which may contribute to identify you and/or stigmatise you further. Especially if one is accounting for other traffic carried by the said custom domain.