LAST ERROR: 554 5.7.1

Hello,

I try to send an invitation to a vendor to complete the profile as a translator and I get this back:

Your message(s) did not reach intended recipients.

MESSAGE #1

FROM: xxxxxxx@xxxx.com

TO: xxxxxxx@xxxx.com

SUBJECT: "Complete your registration!"

CREATED AT: 2019-04-01 14:52:11.424

LAST ERROR: 554 5.7.1 <xxxxxxx@xxxxx.com> Relay access denied.

Any suggestions?

Thanks.