alternate email addresses

I have a user who is receiving outside email addressed to in addition to our default format of 'first initial lastname'. I also tested and found that other formats work for this user such as firstname.lastname, etc. I know the latter is one of the default acceptable formats in GW 8 but the one with the underscore is not. During my test, I sent the same email to him and to myself but I didn't receive those alternate formats, while he did. He does not have a gateway alias or nickname set up using the underscore. I am the postmaster of course, and I do have an alias set for my address, but other than that our settings are the same, so where could that be coming from?