host mx.tb.ukmail.iss.as9143.net [220.127.116.11] SMTP error from remote mail server after RCPT TO:<>: 550 5.1.1 MXIN501 mailboxunknown ;id=lb7LjVL16SCIjlb7LjCYOI;sid=lb7LjVL16SCIj;mta=mx3.tb;d=20200617;t=184051[CET];ipsrc=18.104.22.168;
(I've taken out the email addresses)
However, the email has been delivered and I have received it. I don't think it happens with everyone, but then how would I know? What can I do to solve this? Any and all help greatly appreciated.