compose a message

Yandex

https://mail.yandex.ru/compose?mailto=example@example.com&subject=Hello&body=Text

Mail

https://e.mail.ru/compose/?to=example@example.com
https://e.mail.ru/cgi-bin/sentmsg?mailto=example@example.com – не работает

Gmail

https://mail.google.com/mail/?view=cm&fs=1&to=someone@example.com&su=SUBJECT&body=BODY&bcc=someone.else@example.com

I thought I’d try and consolidate the information from several good answers and comments and provide some additional information. Examples first then background and explanation of terminology below.

N.B. All links will fall back to the default account if there is no match

Simple Compose Link

Opens the inbox with a compose window in the bottom right

Default account:
https://mail.google.com/mail/#compose
Specific account by index (second in this example):
https://mail.google.com/mail/u/1/#compose
Specific account by email address (lee@example.org in this example):
https://mail.google.com/mail/u/lee@example.org/#compose
Any account from specific domain (example.edu in this example):
https://mail.google.com/a/example.edu/mail/#compose
From specific delegated mailbox (id abcd1234efgh5678 in this example):
https://mail.google.com/mail/d/abcd1234efgh5678/#compose
From specific delegated mailbox belonging to non-default account (id 1234efgh5678ijkl, fourth user):
https://mail.google.com/mail/u/3/d/1234efgh5678ijkl/#compose

Each example above automatically redirects from #compose to #inbox?compose=new. You can use either URL.

Custom compose links

Include a to address, subject, body, cc and bcc by opening the Compose Message (cm) view directly. This opens the compose message view fullscreen (even without fs=1 parameter). May struggle with some special characters in the URL.

Default account:
https://mail.google.com/mail/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com
Specific account by index (second in this example):
https://mail.google.com/mail/u/1/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com
Specific account by email address (lee@example.org in this example):
https://mail.google.com/mail/u/lee@example.org/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com
Any account from specific domain (example.edu in this example):
https://mail.google.com/a/example.edu/mail/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com
From specific delegated mailbox (id abcd1234efgh5678 in this example):
https://mail.google.com/mail/d/abcd1234efgh5678/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com
From specific delegated mailbox belonging to non-default account (id 1234efgh5678ijkl, fourth user):
https://mail.google.com/mail/u/3/d/1234efgh5678ijkl/?view=cm&to=morgan@example.com&su=SUBJECT&body=BODY&bcc=dakota@example.com

Complex compose links

Anything you can put in a mailto: link can be passed to Gmail. You need to URL encode* the mailto link first. This takes care of accented letters and other special characters that URLs can choke on:

mailto:morgan@example.com?subject=SUBJECT&body=BODY&bcc=dakota@example.com
  v  v  v  v  v  v 
mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com

Default account:
https://mail.google.com/mail/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com
Specific account by index (second in this example):
https://mail.google.com/mail/u/1/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com
Specific account by email address (lee@example.org in this example):
https://mail.google.com/mail/u/lee@example.org/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com
Any account from specific domain (example.edu in this example):
https://mail.google.com/a/example.edu/mail/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com
From specific delegated mailbox (id abcd1234efgh5678 in this example):
https://mail.google.com/mail/d/abcd1234efgh5678/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com
From specific delegated mailbox belonging to non-default account (id 1234efgh5678ijkl, fourth user):
https://mail.google.com/mail/u/3/d/1234efgh5678ijkl/?extsrc=mailto&url=mailto%3Amorgan%40example.com%3Fsubject%3DSUBJECT%26body%3DBODY%26bcc%3Ddakota%40example.com

*More information on URL Encoding / Percent Encoding on Wikipedia. At the time of writing, urlencoder.io allows you to encode online and also has links to blog articles on how to URL Encode using a few common languages.

Background and Definitions

You can log in to multiple Gmail and/or Google Workspace accounts.
The first account you log into in a fresh browser becomes your default account.
The default account can vary for you if you use different browsers or browser profiles. It can also change if you sign out, remove all accounts and sign back in in a different order.
If you sign in to multiple accounts they are indexed in order of sign in. The default account has an index of zero. Navigating to https://mail.google.com/mail/#inbox will usually redirect you to your default account (index zero) if you are signed in to more than one account: https://mail.google.com/mail/u/0/#inbox
You can specify a Google Workspace domain by inserting /a/<domain> before /mail (whereas user index /u/<index> goes after /mail). This can be useful when you are providing a link internally in a company and you want the link to compose a new email in their work account (regardless of what order they signed in to their various accounts with)
You can compose a message from a delegated mailbox only if you know the delegation ID (and the user index of the account with delegate permissions if not the default account):
https://mail.google.com/mail/d/ALGkd7w86mZOVigdDwK63uDpfMPlA2GJN7qSc4P7idKz7ffqTN0L/#compose OR
https://mail.google.com/mail/u/0/d/ALGkd7w86mZOVigdDwK63uDpfMPlA2GJN7qSc4P7idKz7ffqTN0L/#compose OR, for user number 4,
https://mail.google.com/mail/u/3/d/ALGkd7w86mZOVigdDwK63uDpfMPlA2GJN7qSc4P7idKz7ffqTN0L/#compose

URL to compose a message in Gmail (with full Gmail interface and specified to, bcc, subject, etc.)