At one point in the past, a spammer/attacker created nearly 1 million Apple iCloud accounts using Mailsac (same happened with Twitter unfortunately). So it is likely Mailsac (and other disposable/testing providers) are going to be blocked.
If you contact support, you have some options that will clear things up.
1. We can put your inbound traffic on a new separate node, which should help. This may require adjusting some of your DNS settings.