Guess what ?
Apparentely yes, there is little a issue that may lead to confusions by having a dot in the username.
This is the response I got from xName :
In the DNS world, the SOA 'Start of Authority' contains email address of DNS administrator. This email is written replacing the '@' with a '.'.
So [email protected] is replaced with firstname.lastname.mydomain.com in the DNS configuration (this is not dependent of XName, but dependent of DNS system itself). If someone want to contact the DNS administrator, should he contact [email protected], or [email protected] ?
That's why we don't allow dot in email addresses, to have coherent datas in DNS configurations.
This should change in the future, allowing you to setup an email address to be used in SOA different as the one used on your account itself.
But waiting for this to be coded, you'll have to specify a valid email address without a dot before the '@'...