I was looking at their policies, and I am worried about the "Forbidden Services"

Bulk email providers need pretty tight and aggressively worded ToS by necessity because they're a target for spammers & abuse. The owner of mxroute has been around on various forum for years & consistently strikes me as a very reasonable bloke that won't cause you problems if you don't cause him problems.

I have 5 domains... Any idea what "massive numbers" means?

Maybe I'm imagining this so please don't quote me on this but I vaguely recall hearing them answering a question around what constitutes reasonable in the context of unlimited domains as "if you needed a script/automation to create them then you're probably over the line".

[Note that this is purely my impression as long term customer & I have no special insight/connection to them. Legally they can enforce the ToS]