Note: This blog post is relevant for users of any third party email client on OS X.

A few days ago I was contacted by a MailMate user reporting that he could not switch default email client using the popup provided in the General preferences pane of MailMate. It seemed to work, but after a short while the setting was reverted to its previous state. My initial thought was that this had to be some kind of local issue with Launch Services. This is the service taking care of binding file types and URL schemes to applications. In particular, it can bind the mailto: URL scheme to an application and this is what happens when switching default email client.

I was a bit surprised when I realized it didn’t work for me either. We quickly established that it was a problem on El Capitan only and that it didn’t matter if Apple Mail or MailMate was used to switch default email client. Further experimentation revealed that the problem was most likely not related to MailMate at all.

It still seemed a bit unlikely to be a problem affecting all El Capitan users since googling for the problem didn’t reveal much. There had to be some common denominator. Therefore I searched for users not having this issue via the MailMate mailing list and via Twitter. But I didn’t find anyone.

I currently believe it is a general issue on El Capitan. I’m not quite sure why it hasn’t gotten more attention, but it’s probably a combination of the following:

  • The delayed reversion to the previously default email client.
  • Most often the switch reverts, but it appears that some times the switch does stick.
  • The first switch (clean install or after resetting the Launch Services database) seems to always work.

So, a workaround does appear to be to reset Launch Services, but it’s a bad workaround since it resets all of your custom file type and URL scheme bindings. I don’t currently know any other workarounds other than to keep on trying until it sticks. A special case is if you are trying to switch back to Apple Mail. In that case it also works to uninstall your current email client.

I’ve reported the issue to Apple (rdar://23123392) and hopefully it’ll be fixed soon. I won’t do more than that unless someone comes up with a workaround which I can use to make the popup in the MailMate preferences pane work reliably. If you want to experiment then it might be useful to use the command line hack I created for my mailing list post.

Update November 23rd: A user reports that the workaround described here worked for him. I haven’t tested it myself.