gmail doesn't care about periods, email@example.com and firstname.lastname@example.org both will go to the same place. however i'm registered on the forum as email@example.com while my "official" email address is firstname.lastname@example.org and when i reply to a post via email i get:
Your reply was sent from a different email address than the one we expected, so we’re not sure if this is the same person. Try sending from another email address
which i think is related to the above (i've had a similiar issue on list-serves). but if i try and change my registered email address in my forum account settings to include the periods i get:
There was an error changing your email address. The address is already in use by a staged user.