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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?

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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.

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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).

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    It is quite common for organizations to have spam filters which expressly ditch the message silently. In some situations, generating bounce messages is prone to abuse; look up "blowback" or "backscatter" – tripleee Aug 23 '17 at 11:30
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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)

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