Similar thing happened to me as well. I added another domain for a completely different project and their spam detection locked my account completely, including my main project I've been using for a long time before without any issues.
Got account unlocked after six hours of downtime though, but I changed email provider since then.
I had this and the whole account was locked too after adding a "suspicious" one. But the delivery/reception was still working for other domains, only config changes were disabled.
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.
Got account unlocked after six hours of downtime though, but I changed email provider since then.