Q: how to stop an outgoing mail in Mac mail

You should never rely on that stopping the sending of an email message. The actual send will complete before the indication on the screen indicates success.

Yes, apple mail always had this feature (the X in the progress bar to stop outgoing mail) until El Capitan and I'm missing it already.  BOOO to the repliers here that say it didn't work, or "mail goes out so fast that it wasn't of any value".  I say BS to those replies.  In case you haven't heard, Apple Mail now (since Yosemite or maybe Mavericks) has had the ability to send out really large attachments (I think up to 5 gb).  If you are sending an email that takes several minutes to go (like lots of pictures) you could, until El Capitan, hit the "X" and stop the long upload.  I really did work very well.  Now we seem to have lost that feature and we want it back again!  No, using thunderbird or another mail app doesn't fix the problem.  Just because some of the repliers on here didn't use it, that doesn't mean it wasn't a great feature.

spydie2 wrote:

 

Now we seem to have lost that feature and we want it back again!

Then you need to tell Apple. This is a user to user support forum, and Apple does not participate here. To tell Apple "we want it back again!", you need to go to the feedback page http://www.apple.com/feedback and click on the appropriate subject area.

I would suspect that most email clients are capable of sending messages with attachments of any size. But most email servers are not going to accept attachments over about 20 MB.

You don't understand how it works.  It's not dependent on the receiver accepting a large email like in times past.  If the email is too large for the receiver client/isp, then it sends a link to be downloaded.

Chris, I did report it to Apple.  Of course we never hear anything about our reports, but maybe it will show up again in the next update... and maybe not.

That is always possible. Yes, the feedback information is all read, but they indicate right there that you will not receive a reply. But that is the only method to provide them with information on enhancements you will like to see, things changed, or deleted features returned.

I understand exactly how it works. I have written email clients.

You are talking about using Mail Drop. This is not anemail standard. It is service provided by Apple's Mail client and iCloud. Mail replaces your attachment with a link to your attachment. Mail Drop limits - Apple Support

You can do the same thing with Dropbox, Box, etc.

Yes, you're right, that's how it works, but it's a lot easier than using my dropbox because it's automatic.

Let's hope the Apple developers take a hint from this thread... We users want to be able to stop an outgoing message. Please put the "X" back in.

Have you tried opening the Activity Window and see if the x is there?

I've never had a use for it except long ago when an email got stuck. The Activity Window had a stop button, then. Everything I send goes to fast for me to see if there might be one.

If you want this feature you should stop using Mail and instead use mail.google.com and enable the "undo send" feature.