Exchange 2010 - Allow authenticated user to relay as any email address?

I have a vendor that sends notification emails from job applicants through our Exchange 2010 server. Vendor service connects and authenticates as donotreply@ on a separate receive connector on port 587. The vendor service then says the email is from the applicants email address, Exchange says the client doesn't have permission to send and dumps the message. How can I allow an authenticated user to send as anyone?

*,DOMAIN\donotreply,authenticated
>,235 2.7.0 Authentication successful,
<,MAIL FROM: <applicants_email_address@their_domain.com>,
*,08D3D15AAC2344AA;2016-09-02T15:31:08.310Z;1,receiving message
>,250 2.1.0 Sender OK,
,RCPT TO: <LEGIT EXCHANGE USER>,
>,250 2.1.5 Recipient OK,
<,DATA,
>,354 Start mail input; end with <CRLF>.<CRLF>,
>,550 5.7.1 Client does not have permissions to send as this sender,
<,QUIT,
>,221 2.0.0 Service closing transmission channel,

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