I did the next steps and they worked perfectly on my computer... however, I don't know why but it's not working on the computer I need to implement this. May be it's going to work for you too.
1. Configuration > Configuration > General Settings, section domain alias, turn it to yours (example: if your address is firstname.lastname@example.org, set this to gmail.com).
2. Configuration > Users > Users, select one user (the admin, for example) and go to their Preferences tab. Set their messaging alias to your account alias (example: if your address is email@example.com, set this to myaddress).
Note: in this step, if you save and your address turns to myaddress2, it will be because you declared first the incoming mail server and Odoo created an alias for it. You have to delete that alias from Configuration > Email > Aliases. After removing that row, you will be able to save the user with that alias and Odoo won't turn it to myaddress2.
3. Create your incoming mail server. Configuration > Technical > Incoming Mail Servers, create a new one. (Let's put an example with IMAP: set the name: whichever you want, server type: IMAP server, server name: firstname.lastname@example.org -if your address is a google one-, port: 993, SSL/TLS: True, user: email@example.com, password: pwd_of_your_account). Click on Test & confirm and it will be configured.
4. Now go to Configuration > Automation > Scheduled Actions, and edit the Fetchmail service row. Set the attributes Interval Number to 1 and Interval Unit to Minutes, to check if there are incoming mails each minute (this way you will see if it works faster).
It worked for me, but now, I can't understand why ithe same steps are't working on other computer... If anyone could give me a hint, it would be very appreciated.