STARTTLS Problem With Office365 and SecureMail

Question

Hi all,

We have been using Rebex Secure Mail for the past 8 months sending emails through Office 365. We connect directly to our instance (ie. customerdomain.mail.protection.outlook.com) on port 25 and use SslMode.Explicit to start TLS. As of a couple days ago, the connection start throwing an exception saying "Connection was closed by the remote connection end".

Here is the code we are using:

smtp.Connect(outbound_smtp_server, outbound_smtp_port, SslMode.Explicit);

We have opened a ticket with Microsoft but I figured I would also ask the question here. From packet sniffs we can see that Rebex issues the STARTTLS command and receives "220 2.0.0 SMTP server ready", however once Rebex starts transmitting the encrypted traffic we get disconnected.

Any help would be very appreciated. Thanks!

Answer 1

I just realized there was a new version of Rebex Secure Mail released 2-10-2016 available that has fixes for TLS compatibility. We updated it and are testing now. So far it looks like it has fixed the issue.

Comments

Hello, yes you are right. Actually it is the version 2016 R1 of Rebex Secure Mail (http://www.rebex.net/secure-mail.net/history.aspx#2016R1) that fixes the interoperability issues between Rebex and Office 365 TLS implementation. If you come accross any more issues, let us know.