If it's a well behaving e-mail server it will keep trying to send the e-mail... A trick to stop spammers is to block all new connections for an hour. While spammers wont try again. Sadly some legitimate e-mail servers will not try again either :/
Also some e-mail servers wont try your backup e-mail server... Some servers will even give up if they haven't been able to establish a connection within a second. Some developers/admins give zero shit about edge cases and conditions outside their developer machine. Especially if it's a company that buys invoices they will take any reason to drop the e-mail so they can add a reminder fee.
