Mac Mail.app accounts offline after working hours
Couldn't you just set Mail.app to check only specific accounts, then re-enable them when you want to check them? In the Advanced options under Accounts, you can uncheck the box to Include when automatically checking for new messages. If this box is unchecked, this account will be ignored when Mail.app checks for new email. 



The checking/unchecking of this box can be scripted using AppleScript.

tell application "Mail"
   set enabled of account "NameOfAccount" to false
end tell


This script can then be tied to a macro in an application such as Keyboard Maestro.