Disable email protection

I just created an email account on my domain.

a@a.com

I noticed that when I try to send several emails to a@a.com using my gmail account, the last few are blocked with an error "550 5.7.1 Unable to relay (in reply to RCPT TO command)"

I do not want this feature as I anticipate a large number of unknown senders who will send numerous emails to me.  I cannot add all of them to a safelist.

I tried to disable Spam filtering at the FOPE panel at Mail Control.

I logged in as B@a.com (i.e. another user), clicked Administration -> User -> Add User

I typed a@a.com

then it says The e-mail address already exists.

Please help