Lynda,
It's always possible, however it is unlikely.
To prevent brute force attacks ListMail delays 8 seconds after every failed login attempt. It also has an option to email you with the password that was tried and the IP of the user. I recommend enabling that option, "Notify when a bad password is entered", then if you see someone trying to guess your password you can change it to something very complicated and uncrackable. (ie. 8-12 random characters)
The process -could- be improved a bit further, for example by allowing X tries then banning the IP for a period of time; however, we haven't had any problems with the existing method yet.
If you forget your admin password in the future please visit this post:
http://listmailpro.com/forum/index.php?topic=502.0Regards