The following custom Sieve script can be used to make a copy of every email a user sends or receives.
In the example, the user is user@domain.com and the copy of the .MSG file is saved in the spyfolder directory.
if envelope :contains ["from","to"] "user@domain.com" { fileinto "c:\\spyfolder"; keep; }
Note: Be careful to monitor the size of the spyfolder as the message files could accumulate and fill the disk drive!