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!