revert of this commit
https://github.com/gpc/grails-greenmail/commit/869af4c3515a6f0068d0bd9d2ac76d13bf95a7cf

removed stop() method

https://github.com/gpc/grails-greenmail/issues/5
start() method become synchronized and marked with @Override
null bean error fix
https://github.com/gpc/grails-greenmail/issues/6
Revert accidentally change