It seems they do that on a TLD basis, for example if you add the domain "spammer.xyz", it will probably lock your account because ".xyz" TLD is considered risky.
They support always fix the problem, but we're always locked a few hours, this is super annoying.
As a B2B SaaS, our opportunity cost for support (and potential for lost goodwill) due to failed notifications is high, and switching to Postmark noticeably moved the needle. Time is our most valuable and scarce resource, and peer recommendation is our primary marketing device, so the price increment for switching to Postmark is easily justified. Their deliverability and support are stellar compared to Sendgrid, Mailgun, and SES.
Two thumbs up
See also: Fastmail over G suite for your mailboxes and calendaring.
I do wish they would let you choose the region where the server is located though, like Mailgun does.
Got account unlocked after six hours of downtime though, but I changed email provider since then.