Since Parallels Plesk Panel 9.x, MailEnable 5.x is supported. MailEnable Standard 1.986 and MailEnable Standard 4.x can be safely upgraded to MailEnable Standard 5.x. Your current settings will not be affected by upgrading to MailEnable Standard 5.x.
We recommend performing a backup using the MailEnable backup tool before starting the upgrade process.
For information about upgrading to MailEnable 4.x, see this article:
6961 Can MailEnable Standard 1.986 shipped with Plesk be upgraded to the 4.x version?