The funny thing is, on the real internet with SMTP (so, not in your case but commonly), that's actually true; I once worked at a company that sent a massive amount of email to customers with our own postfix clusters, and it turns out things like "MX was down for a few hours" or "customer's email system rate limited us" - or rarely even "our servers are overloaded and trickling mail out" - are real things and sometimes emails do get held up for hours or days.