I am hosting 3200 email domains and 14000+ users on a single machine with two processors. The mail server I am using is SmarterMail 2.
However the CPU usage and memory usage are in a very low level, my cliets experience SMTP delays and always SMTP connections are failed.
Any comments on this issue ?

What are the exact specs?
That many active users and domains on one machine sounds a little mad tbh.

Intel Dual Xeon 2.8GHz with 1GB RAM running windows 2003 standard edition.
Maximum concurrent web client is 100, maximum concurrent SMTP sessions is 120, Maximum concurrent POP3 connections is 100.

It could be any number of things, but I can't understand why you would want to run that number of email clients through a windows server..

I can tolerate about the number o domains and users. If you mention that, I may add another machine, but, what is threshold number ? I mean, what is the maximum ?

As I said before, the CPU and RAM usage is too low even in the peak times and this makes me confused that so why the SMTP connections should be this much slow ?

You are most likely having disk subsystem issues. With that many mailboxes on the server, you will need at least two seperate physical hard disk. One disk should be dedicated to the OS / spool / logging, and the other disk for the storage of user mailboxes. If you open performance monitor, you will likely find that your average disk queue length on the server goes up quite high. This value should be under 1.

Another possibility is that you need to increase the number of SMTP threads available to SmarterMail's SMTP service. This is configured in the SmarterMail admin.

Something else to check is that if you have catch-all accounts configured on your domains, it's probable that your domains are getting dictionary attacked and accepting a bunch of spam. This, in turn, reduces your available SMTP threads and increases the strain on your disk sub system. Deleting any catch-alls that are only catching spam is adviseable.

What is the proposed number of SMTP threads for this configuration?

try to increase smtp threads and monitor the server usage.... every situation is different... you need to test and tweak/optimise