According to the Kerio Mail server 6 documents (only a brief scan admittedly) their exemplar setup shows Port 465 for SMTPS + ONLY set up for secure mail service (paraphrasing), which is the same port specified by my mail carrier.
So, I use 'custom' on TNAS with the domain address of the server {in my case Dynu's smtp server 'smtp.dynu.com'} with the port set as 465 and authentication method of TLS/SSL.
You do have to enter the password for the designated email user name on the TNAS every time before you can run a test (even after it is successfully set up, which is a nuisance but I guess someone at TM feels it a needed security step
- or just bad programming 
).If you cannot see the authentication method expand the dialogue window until the drop-down remains visible (see last point above).
If you have not followed Kerio's advice for secure setup, that is something I cannot help with, unfortunately.
Good Luck.
{And, for those "doubters" who did not like my previous post, I achieved all this by searching the web, reading the content I found and working it out. If you need spoon feeding go elsewhere.}
