I use Opera as a browser and I use Mail as the email client. In the preferences in the Mail program, there is a setting for Default Email Reader. When that is set to Mail, as I want it, the Share menu within other programs allows me to email from within the program, like from within Pages.

However, when the preference is set in Mail for Mail to be the Default Email Reader, after opening the Opera Browser, the preference in Mail is automatically (annoyingly) to be Opera as the Default Email Reader. The Share menu loses it's Email option, and I have to reset the preference in Mail.

My email address is gmail, and my wife has AOL. Our addresses are not associated with Opera; it's a problem with the software.

I have the same problem. OSX 10.10.5.
When I change the default mail reader from within Mail, it will remain so until the next time I open the Opera browser, regardless if I click on an email link or not. Just opening the Opera browser modifies the Mail preference setting so that Opera is now the default email reader.

This is an issue because the option to use Mail disappears from the Share menu, until I go back and change the default reader back to Mail again!