So this is a deliverability issue, and might not entirely be MCs fault (though it is of course their decision). Whether people realize it or not, you don't use us just so the don't have to maintain email servers (that's part of it), but also to make sure that the email they send ends up where it's intended to go (most of the time). That's not just making sure all the SMTP rules are followed, DKIM/SPF/etc. are right, IPs are kept off RBLs, etc., etc. Email hosts also look at the reputation of the email sender in determining whether to deliver to their customers. Email senders have to walk a careful line with the ISPs (Google, Yahoo, Comcast, etc) who provide email to end users. If an ESP sends too much email that tickles a spam filter at an ISP, the ISP will block the ESP, which is a major business hit. Bonus: we rarely know what the thresholds are, and it changes all the time. If we get blocked it can impact much more than one client, and we then have to negotiate with the ISP and convince them that we haven't gone bad, we're policing our clients, and pretty please let us send them mail. We have an entire team of people (10+ folks) that do nothing but manage those relationships. When this happens enough with a single client, even if they aren't technically in the wrong, it's a business decision as to whether the client is worth keeping.
So this is a deliverability issue, and might not entirely be MCs fault (though it is of course their decision). Whether people realize it or not, you don't use us just so the don't have to maintain email servers (that's part of it), but also to make sure that the email they send ends up where it's intended to go (most of the time). That's not just making sure all the SMTP rules are followed, DKIM/SPF/etc. are right, IPs are kept off RBLs, etc., etc. Email hosts also look at the reputation of the email sender in determining whether to deliver to their customers. Email senders have to walk a careful line with the ISPs (Google, Yahoo, Comcast, etc) who provide email to end users. If an ESP sends too much email that tickles a spam filter at an ISP, the ISP will block the ESP, which is a major business hit. Bonus: we rarely know what the thresholds are, and it changes all the time. If we get blocked it can impact much more than one client, and we then have to negotiate with the ISP and convince them that we haven't gone bad, we're policing our clients, and pretty please let us send them mail. We have an entire team of people (10+ folks) that do nothing but manage those relationships. When this happens enough with a single client, even if they aren't technically in the wrong, it's a business decision as to whether the client is worth keeping.