Mail not sending after a failure in previous email to domain

[mailadmin]

I have a problem with mail sending to a domain.
eg
mail 1 is sent to domaina.com from invaliddomain.com
mail 2 is sent to domaina.com from validdomain.com

domaina is rejecting mail1 as there is no dns setup for the
invailddomain.com. mail 2 is never attempted to be sent though.

Why does mail 2 never get attempted to be sent. Nothing in logs,
nothing in network trace.

I think i have sorted out all the servers locally that were using the
invaliddomain, but want to know why once the smtp iis server tries to
send an email that is rejected it doesn't attempt to send other emails
to that domain. I can send email to other domains, within same email
message as domaina and they get sent fine. After removing the failed
emails from the queue folder and restarting the SMTP service the mails
send fine.

Any ideas why iis smtp doesn't send?

Thanks