url-mail-command

url-mail-command is a variable defined in `url-vars.el'.
Its value is

compose-mail

• This variable may be risky if used as a file-local variable.

Documentation:
This function will be called whenever URL needs to send mail.
It should enter a mail-mode-like buffer in the current window.
The commands `mail-to' and `mail-subject' should still work in this
buffer, and it should use `mail-header-separator' if possible.

You can customize this variable.