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I have to write an application which processes and forwards mails which are delivered to a default user, but the original sender must be preserved.

I already know that the Send as permission is required to send mails as any other Exchange user according to the Microsoft KB and also this SF question

But does that also work with users/senders which are not Exchange users?

In other words, this works:    --->  --->

Jane sends a mail to the default user. My application processes the inbox and composes a new message based on the original message and sends it to John with Jane as the sender.

But how about this:   --->   --->

In this case an unknown external sender sends the original e-mail. Can the forwarding application still set the sender of the new message to the original unknown sender?

I fear that this is not allowed because that would be a spammer's tactic (although here it is incoming mail, not outgoing)

(Unfortunately I don't have access to an Exchange server in the foreseeable future, so I can't just test this.)

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this will depend on the configuration of the SMTP server that you are using to send the Email(assuming you are using SMTP), The IP address of your application conputer should be allowed to Relay on the SMTP server for this esenario to work, or you should authenticate to the SMTP server for the Relay to be permited.

Here is an Microsoft KB about configuring Relay on Exhchange servers:

"What Is Relaying? If you've ever received unwanted spam in your mailbox, then you already know what relaying is: using a server to accept and then resend mail to recipients on another server. In the simplest case, connects to the SMTP server at and uses it to deliver a message to Note that this isn't the same as when Alice uses her own organization's SMTP server. A more practical example: say you're on the road with your laptop. You'll probably have a dial-up (or maybe broadband) connection that will assign you an IP address outside your normal network block. If your SMTP server accepts messages from you for delivery to third parties (e.g. addressees not on your own mail server), that's relaying; a server that has relaying turned on will accept mail for recipients in other domains, then attempt to redeliver it."

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