Just had the above message the content of which is below
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:
host gmail-smtp-in.l.google.com [18.104.22.168]
SMTP error from remote mail server after end of data:
550-5.7.26 Unauthenticated email from btinternet.com is not accepted due to
550-5.7.26 domain's DMARC policy. Please contact the administrator of
550-5.7.26 btinternet.com domain if this was a legitimate mail. Please visit
550-5.7.26 https://support.google.com/mail/answer/2451690 to learn about the
550 5.7.26 DMARC initiative. c15si5549153ejz.158 - gsmtp
The problem is that the email address mentioned is not one that I recognize or in fact have sent anything to (I've checked all my sent items).
I haven't opened the attachments.
Is it spam?
I suspect your email address has been spoofed by somebody and a bounce has been sent back to the purported sender, you.