What TLS does Postmark support and use?

Postmark requires a TLSv1.1 or higher connection to send.

That said, right now, our TLS support for outbound messages we send is opportunistic - we'll use TLS if available, but fall back to unencrypted if it's not. Customer cannot control the TLS version being used.This can be shared with the customer.

Inbound messages also support Opportunistic TLS (related case).

Not public information:

We'll use, in order of strongest → weakest:

• TLSv1.2

• TLSv1.1

• TLSv1

• SSLv3