You press Send and immediately notice that the attachment is missing. In Gmail, Undo can stop the message during a short cancellation window. Once that opportunity has passed, deleting your own sent copy does not remove the recipient’s copy.
Set the window before you need it
In Gmail on a computer, open Settings → See all settings. Find Undo Send, choose 5, 10, 20 or 30 seconds, then save the changes at the bottom. These are the choices described in Google’s computer instructions; do not assume another mail app offers the same control.
Try a harmless message addressed to yourself. Immediately after sending, look for Undo beside the sent confirmation and select it while available. Inspect the message that returns for editing. Practising with an empty test message is more useful than discovering where the control is during a real mistake.
A longer window gives you more time to notice an error, but it is still measured in seconds. Changing the setting later does not reopen the cancellation window for an earlier message.
When the opportunity has passed
Read the actual sent message before writing another one. For a missing attachment, a single clear follow-up such as “The packing list was missing; it is attached here” is usually easier to understand than several overlapping corrections. Confirm the address and open the intended file first.
If information went to the wrong person, handle that specific disclosure through the appropriate process. Do not rely on removing the message from Sent or asking Gmail to undo it hours later. The useful habit is to check the recipient, attachment and wording before Send, with Undo as a brief extra chance.