This is very superficial, you'd better look for a howto on www.tldp.org
about mail servers. what I know is:
- To send email you need one daemon, to receive is another one different. Sending requires smtp. Receiving requires pop3.
- To send email, you can just start the server when you need. To receive email, you'd better be online all the time.
- If a machine tries to send email to you and fails, it will try another more times, until it gives up. These timeouts depend on every sysadmin. If the whole process fails, you won't receive that email.
- Keeping a pop3 daemon is a more serious thing, as it relies on a kinda "24/7" hardware.
- Keeping a sendmail server is also a serious thing, because you can be acting as a mail relay to spammers, if misconfigured. Be very careful and study deeply the issue.
- Keeping a sendmail daemon online ONLY when you need to send mail is a more feasible thing. But even so, there are security issues.