If you address an email to a non-existent address then you will get a genuine email delivery failure message in return.
That will make sense as you had previously sent out the message that failed.
Usually it just means a spelling mistake but can mean the recipient has deleted that email address or simply that their email mailbox is full.
However, you may also get such delivery failure messages about messages that you didn’t send.
These are usually phishing scam messages and there will be an attachment to download (filled with malware) or a link to click to get you to input your login and password.
These failure message usually have large chunks of technical gobbledegook such as
host mta5.am0.yahoodns.net [22.214.171.124]
Delay reason: SMTP error from remote mail server after pipelined MAIL FROM:<firstname.lastname@example.org> SIZE=6745:
421 4.7.0 [TSS04] Messages from 126.96.36.199 temporarily deferred due to user complaints – 188.8.131.52; see https://help.yahoo.com/kb/postmaster/SLN3434.html
The messages sometimes do not say the message has been rejected, but that it has been delayed or is held in a queue. Makes no difference – it’s just a scam.
Do not click anything or download anything from such email messages.
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