Sorry for the delay on answering this if you haven't already found the answer. Unless you're trying to manipulate the SMTP strings when incoming, I would recommend simply using a fastL4 and use no persistence, allowing LTM to just forward along the traffic. LB methods would still allow spreading the traffic out. I'm only stating this because most external mail gateways are already scrubbing SPAM/Reputation/DKIM/SPF anyway so why add a new layer of complexity.
However, prior to 11.5, this was possible with iRules
With 11.5 and up, there is an SMTPS profile and this should get you where you want to be.
Also, while CURL is super handy here, also use Openssl s_client to validate consistence. I've found discrepancies between testing and Openssl seemed to reproduce a better client connectivity representation for TLS.
$ openssl s_client -connect mail.example.com:25 -starttls smtp -debug