On: Freenode has blocked the Mibbit client permanently.

[ankhmoop]

Quite a few registration forms do block webmail addresses for reasons of abuse.

[axod → ankhmoop]

Registration forms that were built in the 90s, agreed.

[ankhmoop → axod]

No, I regularly find modern registration forms that won't accept mailinator or even hotmail addresses (and your service is probably more akin to mailinator than hotmail).