can't email to certain addresses

[Kathy]

All email to addresses I have used many time now is returned with "5.1.0 - Unknown address error 550 - Mailbox not found." My emails to other addresses have gone through with no problem. Why just this address?

 

[Guest]

All email to addresses I have used many time now is returned with "5.1.0 -
Unknown
address error 550 - Mailbox not found." My emails to other addresses have
gone
through with no problem. Why just this address?

Perhaps because the people you are trying to send email to no longer have
accounts there. Can you contact them by some other means and ask them
if they now have accounts elsewhere?

Another possibility is that their email provider has changed their rules on
whether they accept addresses containing spaces or CAPITAL LETTERS.
See if it makes a difference if you try similar addresses with none of
those.