I think you either forgot the apt-get update to get the old packages into the cache or you have a diffrent problem.
- if you have another version then 0.60.0-2 you have still the wrong one. Remove it - apt-get update && apt-get install courier-maildrop and verify again.
If you got it you can hold the package (i would use aptitude instead of apt) so it keeps the current version until you say so. Its
aptitude hold courier-maildrop