Proof of delivery, as far as Gmail will give it

Gmail has no delivery receipt. When a message lands in your Sent folder, that means one thing: Google's servers accepted it and took on the job of passing it along. If that job fails, Gmail writes to you. If it succeeds, Gmail says nothing at all. So the honest answer to whether an email was delivered is built from two pieces of evidence, a bounce you did not get and a sign of life from the other side, and it helps to know exactly what each one is worth.

What Sent actually means

Email moves between servers under SMTP, and the standard that defines it, RFC 5321, is specific about the moment of handoff. A server that accepts a message is accepting responsibility for delivering or relaying it, and if delivery fails after that point it must send a failure notice back to the sender's return address. That is the whole of the promise. Nothing in the protocol obliges anyone to confirm success.

It also explains why bad news can arrive late. A temporary failure, such as a busy server or a full mailbox, gets retried rather than dropped. RFC 5321 says the retry interval should generally be at least 30 minutes and the give up time generally needs to be at least four to five days. A bounce that shows up the morning after you sent something is the system working as designed.

Reading a bounce in ten seconds

Google's guide to fixing bounced or rejected emails says a failed message produces an automated email from Mail Delivery Subsystem, sent from mailer-daemon@googlemail.com, usually with the subject “Delivery Status Notification (Failure)”. The useful part is the code inside it. Google publishes the full list in its Gmail SMTP errors and codes reference, and a handful cover most of what a one to one sender will see.

• 550 5.1.1: the account does not exist. Check for a typo, then confirm the address with the person another way. Sending again will bounce again.
• 550 5.2.1: the account is inactive. The person may have left the company. Find their replacement rather than retrying.
• 452 4.2.2: the recipient's inbox is out of storage space. This one is temporary, and the message can go through once they clear room.
• 421 and 450: the receiving server is busy or the recipient is getting mail too quickly. Wait and let the retries work.

The first digit carries most of the meaning. A code starting with 4 is a temporary condition that may still resolve. A code starting with 5 is permanent, and the address or the message needs to change before another attempt has any chance.

The outcome that never reports back

No bounce means the message was accepted somewhere down the line. It does not mean it reached the inbox. A receiving system can file a message in spam, and spam placement counts as delivery, so nobody tells you. Google's email sender guidelines say messages that fail authentication might be marked as spam or rejected, and only the second of those produces a bounce.

The standard admits a harder case too. RFC 5321 calls silent dropping extremely dangerous and against the long tradition that mail is either delivered or returned, and then concedes the point in plain words.

Dropping mail without notification of the sender is permitted in practice.

If you send from a Google Workspace account, your administrator has one more tool. Email Log Search shows a status for each step of a message's delivery, which can settle whether a specific message left your domain and was handed on. Its post delivery details, such as labels and spam placement, describe mailboxes inside your own organization, and it cannot see into anyone else's.

Why a bounce deserves a cleanup

One bounce is a wrong address. A pattern of them is a problem for your account. Google's page on limits for sending and getting mail lists a specific error, “Messages you sent couldn't be delivered”, which can appear when a large number of your emails go to invalid addresses or bounce from the recipient's server. The page says you may be able to send again after 24 hours, and its advice for avoiding it is to check that addresses are up to date before sending. The sender guidelines make the same point for bulk mail: automatically unsubscribe recipients who have multiple bounced messages.

• Fix or remove the address the same day the bounce arrives, in your contacts and in whatever sheet it came from, so it never returns in a later send.
• Treat a 5 code as final for that address. Treat a 4 code as a reason to wait a day, not to resend at once.
• When a role changes hands, ask a colleague of the person for the right contact rather than guessing at a new address pattern.

What a tracker adds, and where it stops

A tracker's sent status means the same thing as Gmail's Sent folder: accepted, nothing more. What a tracker adds is positive evidence from the other side. A confirmed open, a click or a reply shows the message reached a person, which is the confirmation Gmail never gives. The absence of those signals is still not a verdict, because plenty of people read mail without loading images.

Fast Mail Tracker keeps those layers apart. Its signal ramp starts at sent and moves only on a confirmed human open, a click or a reply, while automated fetches are kept on the record without being counted. When it sends each recipient their own copy and Gmail refuses one outright with a permanent error, that address goes on a suppression list so Fast Mail Tracker never mails it again, and the rest of the group still gets their copies. A failure that comes back later arrives the usual way, as a Mail Delivery Subsystem message in your inbox, so that is still the first place to look before reading anything into a silence.

Fast Mail Tracker is in early access while Google's security review runs, so places are limited. If you want the positive half of that evidence on your own sends, use the install button at the end of this article.