Configuring Authentication for Sending Client

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How do I configure to require the sending client to authenticate?

I have nullmailer on a Pi, and I need to allow a few other applications on
other machines over the Internet to send out via this Pi.

Thanks.

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o authenticate?<div><br></div><div>I have nullmailer on a Pi, and I need to=
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 out via this Pi.</div><div><br></div><div>Thanks.</div></div></div></div><=
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