Can someone be very kiind to guide me setting up my email?

You need to set up your MX records (and should set up reverse DNS)

You can read about setting DNS records in the linode manager here http://library.linode.com/linode-manage … de-manager">http://library.linode.com/linode-manager/configuring-dns-with-the-linode-manager

MX records let other mail servers know where to send your email, reverse DNS gives your server a name other than something.members.linode.com which is useful for passing spam checks and a whole host of other things.

Once you've done that the reverse dns name will be the mail server name your client needs, the ports will be either 25,465 or 587 for smtp (465 and 587 are used for ssl connections) and the ports will either be 110 or 995 for pop3 or 143 or 993 for imap.

But I am serving a web app with the same domain name. If I reverse it or change the hostname to mail.domain.com, then my web app won't work.

It will still work, and you don't have to change it to mail.domain.com unless thats what you set in postfix.

A records are what websites uses not reverse dns records.

If your not too confident in setting up and running a mail server, an easier alternative may be to use Google Apps to send/receive your email for you, but still using your own domain name such as abc.com. There is a free and paid version of Google Apps, it's fairly easy to setup with the guides on Google's help pages.