Can an email delivery silently fail?

Question

Can an email delivery silently fail ever, with the sender receiving no notification whatsoever about the failed delivery?

Specifically, can silent delivery failures occur for mail sent to a Gmail account:

1. From another Gmail account?
2. From a non-Gmail account?

I once in a while hear some people complain about not receiving my email, especially when I follow up on the email contents with them over phone. At that point they ask me to try resending.

Doesn't the SMTP protocol guarantee email delivery, or at least a failed delivery notification to the sender?

Answer 1

Doesn't the SMTP protocol guarantee email delivery, or at least a failed delivery notification to the sender?

No, SMTP has no delivery guarantees (you can check it yourself in RFC 5321):

There are mechanisms glued on to the SMTP protocol and associated programs for ensuring delivery (DSN, Return Receipts). Note that these themselves are best-effort / mutual cooperation extensions (Most mail clients let you elect not to send read receipts, and some clients can't issue a read receipt. Some MTAs can't/won't issue a delivery receipt.

...

Specifically, can silent delivery failures occur for mail sent to a Gmail account.

Yes, but that's quite unlikely, Google has excessive infrastructure and usually handles it's data well.

I once in a while hear some people complain about not receiving my email, especially when I follow up on the email contents with them over phone.

• It could be just a polite way to say that they didn't have time\opportunity to read it.

or

• Your phone's internet connection is not reliable and fails silently or your operator filters\blocks your traffic (i.e.: Verizon faces lawsuit over email blocking).

Answer 2

SMTP does not require a bounce back, although it appears Gmail might do it:

There is risk involved with showing what responses a server gives although the bounceback is generally considered enough of a benefit to overlook the risk.

Simple fact of it is: most "undelivered" emails are in one of Gmails folders (In my case I have two Spam folders, one called Spam and one called Junk)