Title sums it up nicely. This problem is odd though; I know it is not an issue with networking (so I've port forwarded correctly etc.) because I can run a vanilla server and everyone can connect fine. When I run my Tekkit Classic bukkit server though, I can connect via localhost but my friend gets "Failed to login; bad login" and my server log is
[INFO] /88.35.654.33 Lost connection
Server.properties is configured correctly, and I'm running in online mode.
What could be the problem?