Your Ironport is failing the set up an encrypted TLS session for mail transfer.
Possibly, either your Destination Controls or HAT Policy is mandating encryption, or the remote end is mandating it.
You need to have a Certificate installed and configured in order for incoming SSL/TLS to work. This may be self-signed, but if it is then a remote side cannot have a policy of verification.
If you have a Destination Control mandating verified encryption to a remote domain, and their certificate is expired or has an invalid chain, then you will not be able to set up TLS. Similarly, remote sites mandating validated certificate will not be able to connect to you if you have an expired or self-signed certificate.
If you have 'encryption preferred', as most domains so then it should fall back to an unencrypted link and so your email will still flow.