After reading the article on
Webmail Integration in Opera 10I became interested in searching for a similar function for my opera 9.6 and found the following:
In Tools> Options> Advanced> Programs, select the “mailto” protocol, click “change”, then select “Open in another program” and specify the path to the opera.
The most interesting is written in the line "Parameter"

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Here is a brief list of popular webmail services and a sample of the parameter for manual configuration for any other service.
Gmail:
http://mail.google.com/mail/?view=cm&fs=1&to=%t&su=%s&body=%m&cc=%c&bcc=%b
Hotmail:
http://hotmail.msn.com/cgi-bin/compose?To=%t&Subject=%s&Body=%m&Cc=%c&mailto=1
Yahoo Mail:
http://compose.mail.yahoo.com/?To=%t&Subject=%s&Body=%m&Cc=%c
Yandex:
http://mail.yandex.ru/compose?mailto=%t&subject=%s
Manual setting:
http: // YOUR_URL_HERE /? To_Field =% t & Subject_Field =% s & Body_Field =% m & Cc_FLield =% c
Taken
from here .