It's not whether the account is valid.
It's whether the name and email address you've supplied to Zen Cart is accepted by the mailserver you've told Zen Cart to connect to in order to transmit the email message.
That particular error message is usually rooted in one of two situations:
a) you've supplied an invalid email address (even though you think you haven't)
b) the mailserver that your webserver is using to do the email delivery ... is checking the address and rejecting it instead of accepting it for sending ... and in that case you'll have to find out from the mailserver what's the exact reason why it's rejected it.