Hi @Deleted
As the Gmail message says, this particular address has been used to SPAM in the past.
If you haven't already tried (you probably have), click onto Report not spam and see if that helps.
Might be one to log with Google to find out what they can do for you if the above doesn't work, and if you have tried putting the address/domain onto your safe senders list?
Best wishes!
Cheers
Damien