As of InterWorx version 6.7.1, SNI is possible for mail sending over ports 587 and 465.
On new installations of both IW6 and IW7, this is enabled by default.
On existing installations, navigate to System Services > Mail Server > MDA Settings. Click the “Convert to Dovecot” button.
Once Dovecot is enabled, a new option will show in the side menu called MSA Settings. Click the “Convert” button. This will enable the Dovecot MSA, which allows for SNI over 587 and 465. Make sure the port(s) that should be used are checked.