With Mail, Apple provides a standard mail program in macOS that is usable in and of itself. After people in my acquaintances and work environment complained about faulty file attachments (in detail, it was mostly about pictures that I had sent via my Gmail account), I looked around for alternative programs. After Thunderbird for macOS is quite a bummer in my opinion and I've never warmed to Outlook that much, it finally became air mail 3.
Great email client
Airmail costs around 10 euros in the App Store, but I don't know of any e-mail program that can handle Gmail and other accounts so cleanly. Now the problem was: How do I make macOS use Airmail by default when I click on an email link? This may not be that easy ;) But first things first! By default, you change the default mail program on macOS but Yosemite directly via the Mail app. To do this, proceed as follows:
1. Start Apple Mail
Opens Apple Mail from Finder or Spotlight search.
2. Go to settings
Now open the Mail settings. To do this, you either press the key combination cmd + , or use the menu in the menu bar Mail -> Settings.
3. Select default program for e-mails
In the settings of Mail you will find it directly in the tab General the point Default email program. In the drop-down list behind it, macOS lists all installed email clients. Select the program of your choice here and close the options and mail.
Default mail program under macOS: Fix problems
In the future, clicking on an email link will no longer open Apple Mail by default, but the selected client - at least in theory ;) That just didn't work on my Macbook and I still ended up in Apple Mail. After the threatening raising of my fist in the direction of Cupertino did not solve the problem, I went looking for a solution on the big, wide web. I found this with Medium: Instead of selecting the default client from the list, click on "Select" in Apple Mail. Now you look for the mail program from the list of programs, close Mail and click on an email link to check whether everything is working.
At least for me, this method has led to success - and that after I had previously tried solutions like "Log out of iCloud and back in" etc. I found the installation of Office 2016 or Outlook 2016 as a possible cause in various forums. I have no idea if that's true, but unfortunately I need the Micrsoft suite for work-related reasons.
A more radical solution to the mail problem is available in Apple forum. Open the terminal and enter this command:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user
This will reset the default apps on macOS. For example, if you have a default browser other than Safari you have to repeat this afterwards. At the same time, many affected users report that the selection of the default e-mail client works again.
But that's not what you know from Apple - it just works :-p