Office 365/IIS SMTP relay - FROM address not allowed to relay

I have an old VOIP system that sends voicemails as email links/attachments ...
the FROM address (prefix) is always
How do I allow IIS's SMTP virtual server to relay (to Office 365) with the FROM address constantly changing?
All devices can relay just fine.  
Only this VOIP system is an issue (where I cannot keep the "FROM email address" constant)

Until now, I always used scenario 2 in this guide to successfully setup SMTP relay with Office 365:

Toshiba Strata CIX
Stratagy ES Voice Mail Application
K BAsked:
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Vasil Michev (MVP)Commented:
You can configure it as explained in the "SMTP Relay" section here:

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K BAuthor Commented:
Vasil, you rock!
Casey WeaverManaged Services Windows Engineer IIICommented:
Did this solution work for you? We're running into this issue on a Strata, but using a relay doesn't work because O365 will say that the sender is not permitted (because of the ever changing email address matching a phone number in the FROM field. Any way to get around this?
K BAuthor Commented:
can you send screenshots of your IIS SMTP relay?
Yes it is currently working now
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