![]() ![]() work together to decide the version of TLS Exchange server should use during TLS handshakes. Our previously published 3 part blog posts ( Exchange Server TLS guidance part 1, Part 2 and Part 3) extensively covered how various components like Schannel, WinHTTP. In case there is a failure during communication, Exchange will instead re-attempt the delivery without TLS. Once this version is selected during the TLS handshake - Exchange does not attempt a lower version of TLS/SSL that might also be enabled on the server. For sending mail from Exchange: For outbound email, if the receiving server does not support TLS (does not advertise the STARTTLS Verb), Exchange on-premises and Exchange Online will send email without TLS (provided TLS is not forced on the send connector or outbound connector).Īnother point to keep in mind is that Exchange will always attempt to initially negotiate the highest possible version of TLS which is enabled on the other server.For receiving mail into Exchange: If the sending server does not support TLS, or if the TLS negotiation fails, Exchange Online will still accept messages unencrypted and without TLS (provided the sending server’s configuration allows that). ![]() What happens if a server on your side can only use TLS 1.0 with SMTP? Will sending fail, and if yes, how do you notice TLS 1.0 or TLS 1.1 is the root cause of your email problems? There are different variables that impact this and we will try to mention most frequent scenarios.īefore diving into further details, keep in mind that generally speaking, the TLS implementation in Exchange on-premises or Exchange Online is done opportunistically. We wanted to talk about what this means for SMTP traffic destined to Exchange Online in particular. Also note that Exchange Online will never use TLS 1.0 or 1.1 to send outbound email. This essentially means, soon this deprecation process will be over, and we will no longer accept TLS 1.0 and TLS 1.1 email connections from external sources. The rollout will continue over the following weeks and months. This topic may be super-relevant to you, because as confirmed by the message center post MC229914, TLS 1.0 and TLS 1.1 deprecation started enforcing for Exchange Online mail flow endpoints beginning January 11th, 2021. Head out to our documentation for more details and references if you need a refresher! Also check out this blog entry to see how you can use reporting in Exchange Online to get an overview about the TLS versions used by mails submitted to your tenant. By now you are hopefully aware of the TLS 1.0/1.1 deprecation efforts that are underway across the industry and Microsoft 365 in particular. ![]()
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