Moodle isn't really a mail client kind of application, but there are other ways of getting a message to an instructor, for instance. The regular way would be to go to the participant list in a course, open the profile of an instructor and then choosing the Send a message option there.
If you do want to add a mail option, you could just use the 'anchor' button in the text editor and then add a 'mailto:email@example.com' type address instead of an 'https://www.example.com' one. This will trigger a new mail to be opened from the user's email application.
Agree with Joost recommendation to use the Moodle interface rather than a mailto: link. Why? I use a Mac laptop but my mail is on google. Clicking a mailto: link will call up Mac Mail from laptop and I've not configured Mac Mail and don't plan to.
Your users might be using a PC rather than a Mac and they to might be using an IMAP/Web mail kinda setup for their email. Confusing to users ... plus if your mailto: link doesn't work, they will be communicating with you to 'fix it'. :|
'SoS', Ken
Interesting, Rick. But what launches on students computer?
Using Google?
https://mail.google.com/mail/u/2/[yourgoogledomain]#inbox?compose=new
Replace [yourgoogledomain] with your google domain for email.
That launches a browser window, goes to google, where if you are not logged on it prompts to login, then sends user directly to composing and email message.
Tinkering with that some more one might be able to pass the 'To line' addy!
'SoS', Ken
Shadi seemed to be asking for some techniques, and this is one. Yep, what happens if the student doesn't have an email client? The same thing for many many "mailto" links... nothing.
Of course, another technique is to create a "web form" probably including reCaptcha, and have the students directed to this form when they have questions. Yep, a URL resource would work. I don't know if Moodle can handle these kinds of web page forms, but I would put it external to Moodle, like a school web page.
Or provide both approaches.
Once supported a corp moodle that used an app called MachForms - commercial - but not that expensive.
Gave ability to create forms for all sorts of needs and it could be installed inside moodle code directory and not clash with moodle - not even with git updates/upgrades of moodle. It could be configured to drop/strip out all of it's own navigational links so that the form could be embedded in Moodle.
https://www.machform.com/pricing/
Forms code backed up with the moodle code backups and only a separate DB dump of the DB used for machforms needed.
I don't work for them ... and get nothing for mentioning them.
My 2 cents!
'SoS', Ken