Currently if you have the username or alias "email@example.com", it "locks out" the name joeblogs at every other FastMail domain (eg firstname.lastname@example.org, etc). In a couple of months, we plan on "splitting" the domains, so that all names in all domains become totally independent of each other.
As part of this, we have to ensure that everyone using email software to send or receive emails is logging into their account using their full username (eg not just "joeblogs", but "email@example.com"). Any people with incorrect details, will be sent an email detailing exactly what they need to change and by when with instructions on how to make the changes. These changes are generally minor, but will help us achieve a number of long term changes.