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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?
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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?

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can you send screenshots of your IIS SMTP relay?<br />
Yes it is currently working now
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can you send screenshots of your IIS SMTP relay?
Yes it is currently working now

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I have an old VOIP system that sends voicemails as email links/attachments ...<br />
the FROM address (prefix) is always <b>thephonenumber@contoso.com<wbr /></b><br />
How do I allow IIS's SMTP virtual server to relay (to Office 365) with the FROM address constantly changing?<br />
All devices can relay just fine. &nbsp; <br />
Only this VOIP system is an issue (where I cannot keep the &quot;FROM email address&quot; constant)<br />
<br />
Until now, I always used scenario 2 in this guide to successfully setup SMTP relay with Office 365: <br />
<a href="http://o365info.com/smtp-relay-in-office-365-environment/" rel="ugc">http://o365info.com/smtp-relay-in-office-365-environment/</a><br />
<br />
VOIP:<br />
Toshiba Strata CIX<br />
Stratagy ES Voice Mail Application<br />
<a href="http://cdgenp01.csd.toshiba.com/tsd/docum/CIX-Documentation/CIX-MA-PRGM2-VA1.pdf" rel="ugc">http://cdgenp01.csd.toshiba.com/tsd/docum/CIX-Documentation/CIX-MA-PRGM2-VA1.pdf</a>
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I have an old VOIP system that sends voicemails as email links/attachments ...
the FROM address (prefix) is always thephonenumber@contoso.com
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: 
http://o365info.com/smtp-relay-in-office-365-environment/

VOIP:
Toshiba Strata CIX
Stratagy ES Voice Mail Application
http://cdgenp01.csd.toshiba.com/tsd/docum/CIX-Documentation/CIX-MA-PRGM2-VA1.pdf

Office 365/IIS   SMTP relay - FROM address  1235551212@contoso.com not allowed to relay

Office 365 is a group of software plus services subscriptions that provides productivity software and related services to its subscribers. Office 365 allows the use of Microsoft Office apps on Windows and OS X, provides storage space on Microsoft's cloud storage service OneDrive, and grants 60 Skype minutes per month. Office 365 includes e-mail and social networking services through hosted versions of Exchange Server, Skype for Business Server, SharePoint and Office Online, integration with Yammer, as well as access to the Office software. All of Office 365's components can be managed and configured through an online portal; users can be added manually, imported from a CSV file, or Office 365 can be set up for single sign-on with a local Active Directory using Active Directory Federation Services.

Microsoft 365

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.