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I'm working on a migration to Office 365 / Exchange Online for a SMB company which until now has been running a local Exchange 2010 server; we're talking about ~25 mailboxes here.

However, there's a LOB application which sends messages via SMTP to Exchange, it's based on classic ASP and uses the quite old CDO libraries.

I know Exchange Online only accepts SMTP submissions when authentication is used, and only with TLS encryption; however, it looks like CDO doesn't support this: the most it can do is SSL, which Exchange Online used to support until some time ago, but it doesn't seem to support anymore.

There are several discussions lying around the Internet (including this very site and StackOverflow) which suggest using SSL to make this work; however, as I already said, this was only an option until some time ago: now Exchange Online only supports TLS for SMTP submissions.

I'm also perfectly aware of both possible workarounds, i.e. 1) using a local SMTP relay and 2) creating a receive connector in Exchange Online to allow unauthenticated relay from the company's public IP address; however, for reasons that should be obvious, I'd like to go the proper way, and have the application send its messages using SMTP authentication.

I'm looking for a definitive answer here: is it possible now to have a CDO-based application submit messages to Exchange Online? Or, more specifcically, is there any way for a CDO-based application to support SMTP authentication over a TLS connection?

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    • Thanks @MichaelHampton, that's one of the questions I already examined; however, it's specific to AWS (and also it's not really clear about whether SSL is enough or TLS is actually required). I need a definite answer about if this can (or can't) possibly work with Exchange Online, in order to tell some developers "either you switch to more recent libraries or you won't be able to send anything anymore, and please don't blame the migration about this, because it's your fault if you're still stuck with classic ASP".

It makes me feel dirty to say this, but why not keep the EX2010 box and make it a hybrid deployment? You could continue to relay messages from your CDO libraries to SMTP receive on EX2010, then they would go across to O365 via the "on-premises to cloud" connector.

Then you can explain to the business that their old LOB application is causing increased infrastructure costs. Once they update the LOB app, then they get rid of an Exchange server on-premises, saving $$$.

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    • If it turns out that an SMTP relay has to be used, at least in the short term, I'd probably prefer to use a third party SMTP relay like SendGrid or the like rather than maintain the on premises Exchange server just for this purpose.

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