Before attempting to do one of the following, save a copy of your existing configuration files in another local folder.
In the config-<instance>.xml file, you can add an option: <mta><relay address="127.0.0.1" port="25" /></mta> If the <mta> tag already exists in this file (which is normally the case), you just need to add the <relay> tag inside.
The 127.0.0.1 address can be changed to any MX or email server IP, depending on your needs. Note that the email server must be correctly configured to accept all emails sent to it, including with correct domain name, correct user accounts/aliases (some servers offer wildcards to avoid some configuration steps - but with lower security).
A formal description is available in the serverConf.xml file (it is also possible to make the modification in this file so that it applies to all instances).
Note that Campaign does not support authenticated SMTP. You need to configure the mta so that it accepts all traffic coming from the loopback (and only from the loopback to secure it, which should be 127.0.0.1 if used in local or the Campaign server IP address).