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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mvalpredaAsked:
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MAS (MVE)EE Solution GuideCommented:
Hi,
If you want a secure way allow relay only on that particular IP. Detailed step below.
https://www.experts-exchange.com/articles/2666/Allow-relaying-on-Exchange-2007-Exchange-2010-in-4-easy-steps.html

Thanks
MAS
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mvalpredaAuthor Commented:
I read through that and it doesn't quite seem to fit what I am doing.

The vendor is not trying to send emails to an external user, they are trying to send an email to an internal user spoofing an external user.
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MAS (MVE)EE Solution GuideCommented:
This is the commands you have to use
New-ReceiveConnector -Name AllowRelay -usage Custom -Bindings '192.168.1.100:25' -fqdn server.domain.com -RemoteIPRanges <vendorIP> -server <YOUREXCHANGESERVERNAME> -permissiongroups AnonymousUsers

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Then run this command to allow relay on that connector
Get-ReceiveConnector AllowRelay | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "ms-Exch-SMTP-Accept-Any-Recipient"

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Replace 192.168.1.100 with your Exchange2010 IP

This will work for internal and external.
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mvalpredaAuthor Commented:
Will that make it so authentication won't work any longer? They have their side set up to use authentication to send and I'm trying to work with what they already have set up.
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ArneLoviusCommented:
When the connection is authenticated, by default it can only send as that user, there is no mechanism in Exchange to allow an authenticated connection to send as "anything"

This is why MAS suggested setting up a relay connector bound to their exit IP address and to use an anonymous (not authenticated) connection.

You may however have issues if the original senders address is covered with an SPF record.
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mvalpredaAuthor Commented:
Had to have the vendor change one of their email processes to not use authentication.
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ArneLoviusCommented:
excellent to hear :-)
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