You do not need to have the SMTP port - thatis, TCP 25 - open for incoming requests on the machine (e.g. your laptop) from which you're sending mail via a mailclient.
The SMTP port must be open only in a machine which is used as a SMTP server, i.e. to receive mail (and then either forward it to another mailserver up to its final destination, or deliver to a local inbox).
EDIT: If you need to use a SMTP server running on the same machine, in this case you make sure that the SMTP server is configured to accept SMTP connections from localhost only; it doesn't need to accept connections on port 25 from outside.
An open relay can be a big problem, as it can - and will - be abused to send spam. For this reason, most mailservers e.g. Sendmail, by default configuration, do not accept connections from outside.