It seems to me that some email aliases are blocked to use. 

Most aliases I can add without any problems.  But for some, I get " Unable to add Alias Address".

I am sure, that the alias is not in use.

One, for example, is if the alias includes "paypal" (e.g. paypal@domainname).

Why and where do I find I list of forbidden alias contents?

Thanks,

Gabe