Conversion to 7BIT required but not supported

[Dino Edwards]

Hi,

I've got an odd issue on a brand new Amavis install on Ubuntu 18.04. I'm 
getting rejections on certain incoming e-mails. I'm hoping someone can shed 
some light into this:

dsn: . 550 MtaRejected <some...@gmail.com> -> <some...@mydomain.tld>: 
on_succ=0, on_dly=1, on_fail=1, never=0, warn_sender=, DSN_passed_on=0, 
destiny=-3, mta_resp: "550 5.6.3 id=02074-04 - Rejected by next-hop MTA on 
relaying, Conversion to 7BIT required but not supported"

Thanks!