If you want to have mail sent to user@yourdomain.com sent to your server, then you have to have your server registered as yourdomain.com. It's not good enough to just have yourdomain.com as your domain in your server's settings, because no DNS servers know about it

If you have a static IP address, then by all means, make an A record for your domain (I'm assuming you already have a registrar by the language you used).
However, if you have a dynamic IP address then you'll need to set up a dynamic DNS service. I use
DynDNS myself, but there are others like No-IP(.com) out there. There are a few things you might want to know about this though. Firstly, some mail servers block certain dynamic IP ranges (since dynamic IP's are usually used by home users, who are the most liable to have open relays). Secondly, if you have a domain name then you'll need to make a CNAME record to point to your dynamic dns service. The problem with CNAMES is that (last time I checked) they aren't great if you want to have MX backups. If this is just your own server and you don't need backups for when you have an outage for example, then that's not much of a problem.
I hope that answers your question

Edit: No-IP link killed by spam filter
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yes i have a static ip set up in my router. i know i did the no-ip when i had dynamic dns long ago but now i have a commercial account with my ISP and they gave me a static. So how would i go about setting that up for my mail to come to my linux box?