This code is ok with

NT 4 Workstation Service Pack 4
Visual Studio Service Pack 2
Office 97
*-* MAPI.DLL 4.0.993.3 11/17/96 0:00 494688 No 4.0.0.0
*-* MAPI32.DLL 4.0.993.3 11/17/96 0:00 730384 Yes 4.0.0.0

But not with

NT 4 Workstation Service Pack 4
Visual Studio Service Pack 2
Office 97 SR1
*-* MAPI.DLL 5.0.1457.3 11/07/1997 0:00 518688 No 5.0.1457.3
*-* MAPI32.DLL 5.0.1458.49 11/07/1997 0:00 892688 Yes 5.0.1458.49

After running the following test program and try to modify a class you receive the following message

This file is incompatible with the current version of Visual FoxPro
Run 30UPDATE.PRG to update the file to the current version

LOCAL loForm

loForm = CREATEOBJECT( 'Form' )
loForm.ADDOBJECT( 'oleSession','MAPISession')

loForm.oleSession.signon()

loForm.ADDOBJECT( 'oleMessages','MAPIMessages')

loForm.oleSession.SignOff()

loForm.RELEASE

RETURN

DEFINE CLASS MAPISession AS OLECONTROL
OLECLASS = 'MSMAPI.MAPISession'
OLETYPEALLOWED = -2
ENDDEFINE

DEFINE CLASS MAPIMessages AS OLECONTROL
OLECLASS = 'MSMAPI.MAPIMessages'
OLETYPEALLOWED = -2
ENDDEFINE