set up an SMTP relay on the local network which sends to the Office 365 SMTP server

We are migrating from in house Exchange 2007 to Office 365. Our VB based order management app uses SMTP to send emails to customers. Instead of doing a ton of recoding to use a new SSL local component, are there any issues with just setting up a local SMTP relay to Office 365? If we have the local SMTP relay server, than we can stick with port 25 and non-ssl.  The relay would send SSL however to the Office365 SMTP server.

Environment is Windows Server 2008 R2, which the VB based app runs on.
pkromerAsked:
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RoninCommented:
You have the following options.
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Adam BrownSr Solutions ArchitectCommented:
If your application simply doesn't support SSL and you want to deliver to O365 using SSL, you would set up an IIS-based SMTP relay following these instructions: https://support.office.com/en-us/article/how-to-configure-iis-for-relay-with-office-365-eb57abd2-3859-4e79-b721-2ed1f0f579c9

Ronin's link covers options for sending, but unfortunately the link that explains how to set up a relay server for O365 in that site doesn't work at the moment. The link in this comment does.

If you are ok with not using SSL (it *is* okay not to use SSL/TLS with email, just note that packet sniffers can read the email if they happen to be looking at ports or systems that exist in the mail exchange path), but it's best to use SSL/TLS for mail. If you are okay with not using SSL/TLS encryption, you can just configure the application to send directly to O365 by using port 25 and the
domain-com.mail.protection.outlook.com address in your MX records as the server the application sends to, then configure a receive connector in O365 that accepts mail from the IP address your on-prem network uses for Internet and add a whitelist connection filter entry in the spam filter.
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Scott TownsendIT DirectorCommented:
Can the receive connector be setup to use a IP Subnet without listing all of the IPs?  I have a Class C Address Space and the servers/devices sending mail use Dynamic NAT and could be one of about 150 possible addresses.
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Adam BrownSr Solutions ArchitectCommented:
Yes. The connection filter and receive connector can use a Class C range by CIDR notation or as a range of IPs.
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