Thanks for your reply
"impresora@xxx.com" is not the recipient addres, is the printer's mail address (because in Spanish printer="impresora").
So if I want to send an Scan to my teammate Paul, the From address is impresora@xxxx.com and the recipient is paul@xxxxx.com
As you can see in this picture I've made:
So as far as I know, my from and rcpt address are the correctly ones.
About my mail provider, It allows smtp protocol and "mail.xxxx.com" point to smtp, imap and pop. Instead of having smtp.xxx.com and pop.xxx.com we have only one dns mail.xxxx.com and is the nat wich decides depending of the port.
The remarkable thing is if I send an email from any account TO the printer's mail address, It prints it! so that means the printer is correctly connected (at least) to the POP server.