Why is email address banned?

[zepper]

I tried to update my email address in the UCP and when I entered my fastmail.fm address (which is my main address now) I got a message (approx.): "The email address you used is banned. Contact Admin. for more info."

Now Fastmail is a legitimate email and hosting services vendor and my account is PAID FOR! not free like others (and like my gmail address that was accepted, for example) - but you can get free, very limited mailboxes there.

So I'd like to know why fastmail addresses are banned?

.bh.

zepper,

I replied to your email earlier today. I've reset your account's email and you should have no further difficulty with it. Drop me an email if you have any questions, thanks.