I've been researching and found that
The recipients are anything you can send mail (eg mailbox, group, resource, contact etc ...)
The mailbox is the conbinacion of a user directory and a reserved space in a database where information as exchage (email calendar, history, attachments etc ...) are saved.
My question is when I modify add-recipientPermisions to send as;
¿I am modifying the permissions to the address of mail which is stored in active directory?
And when I use add-mailboxPermisions
¿I am modifying the permissions of the area reserved in the database that stores emails?
In both cases, you're modifying the user object within active directory.
add-recipientPermisions SendAs adds the desired user with the 'Send As' permission under the destination user's permission list. You can view this in AD Users & Computers. Open the properties of the destination user and navigate to the 'Security' tab. The user you gave 'Send As' permission to will appear in this list, with the 'Send As' permission.
add-mailboxPermissions actually adds the desired user to the msExchDelegateListLink attribute. You can see this in ADSIEdit by connecting to the 'Default Naming Context', or you can enable 'Advanced Features' in AD Users & Computers, open the properties of the user, and navigate to the 'Attribute Editor' tab.