Email is not instant messaging; it is email.
Let me explain. Masegaloeh is correct with regard to assumptions, but there is one other explanation that you have overlooked: email is queued. Just because a message is accepted for delivery, doesn't mean that it has been delivered.
And that's just the beginning. The message may travel directly to your mail server, or it may travel through a long, winding route of mail servers. Each mail server along the way may have different policies about what is and isn't acceptable. Each mail server may actually choose to queue your message, or worse, decide to return it back to you ala "return to sender".
Here's another way to think of it; the movement of email from the sender to you is very much a rough analog of the Postal service, in that the message is moved from point to point over time, until it finally reaches the end-point, which is your Inbox. But if we run with that analogy, there are some other things as well. Your message has to contend with random labor strikes (server down), washed out bridges and tornados (network connectivity issues), bandits (malicious hackers), terrorists (malware writers clogging a queue with worms), traveling salespeople selling snake oil (spam clogging a queue), the occasional despot (a "spook" agency looking at what you sent), private mail stops run by tin-pot tyrants (unfriendly email administrators/poorly-configured mail servers), and just sometimes, the dog eats the mail in transit (disk failure).
The short, easier-to-remember rule of thumb is: until it's delivered, never assume that your finished with your message. This is why we have delivery receipts in email; it ensures that it arrived to its destination.