Cannot send email to user@localhost - "The Message doesn't have any valid recipients"

Hello,

I am unable to send an email to an address like user@localhost.

When I try to send a message to an address in this format (where there is no dot . after the @), I receive the following error:

“The Message doesn’t nhave any valid recipients.”

My understanding is that this address format (user@localhost) is compliant with RFC standards.

Could anyone please advise on why this is happening and how I can successfully send an email to this type of address?

Thank you.