When a user is created with a colon at the end of his username, for example test: the member name __ is accepted and It gets the usual activation email. It is then possible to confirm the account. But... if you then try to write to that member with another account, the email is never sent, but a strange email appear on the server, sent to member name @ servername, as you can see here.

A colon at the end of a member name provokes a bug in phpbb that transforms the contact email in membername minus colon at the end @ servername.

Inbetween, I've closed the possiblity to have such names accepted on our 3.3.5 phpbb.