SMTP server

Install postfix.

You can restrict mail postings to IP ranges, whick is OK for you as you have a static IP. Otherwise you can use password authentication (SASL).

Remote servers receiving your email won't try to open your SMTP port to verify that you aren't sending spam. They may look your IP up in a database of open relays or known spammers, or check to see that your reverse DNS exists and maps back to your IP (which caker has already taken care of by default). In short, this won't be a problem.

If you only allow your own DSL address to connect, you won't be able to receive any email from other people on your Linode. Presumably you have another server that you plan to get your email on.

You should be aware that your own DSL provider may block outgoing SMTP connections, attempting to force you to use their SMTP server. If that is the case, you may have to move the service to another port.