Secure opening of ports

[David Baron <d_baron@012.net.il>]

I posted before on the idea of opening, say the SMTP port briefly, 
periodically to recieve any email addressed to my server. I had knockd, the 
very cute port-knock daemon recommended. Of course, if I am doing the 
knocking, this will work. A server, SMTP, HTTP, etc., must be always open or 
openable under the right and secure circumstanced.

Two examples are given in the knockd configuration. The first opens of port 
after a sequence of "knocks". The second opens it for a period of time.

1. From an outside machine, say using openSSH, how do I configure to do this 
sequence and open my home machine's SSH port?

2. How might I keep a secure SMTP port available when needed but hidden 
otherwise using knockd's 2nd example? or the brief periodic opening I first 
proposed? Other?