Can't return emails - SMTP Error (550): Failed to set sender

[Funbows]

Hi everyone. I so would appreciate some help on this. I've never had this happen.

When I try and return an email I get this.

An error occurred!
SMTP Error (550): Failed to set sender "funbows@funbows.com" (Administrative prohibition).


Is anyone familiar with this and can help me ?

Thanks so much

Cheryl