The title already says much; I want to send mails with an adress ( firstname.lastname@example.org ) Until we got our Exchange-Server, we were recieving via POP and sending via SMTP. The office (4 People) wants still to recieve and send with this E-Mail. The recieve is easy to solve with the POP-Connector that distributes the mails to the accounts, but how about sending? (All mails should go via Exchange-Server but with source adress: email@example.com)
You will have to create a mailbox on the Exchange server with that address as the primary SMTP address. Then give the users SendAs privileges to that mailbox. When sending a message, they can pick this mailbox from the menu next to "From" on the new message.