Possible options:

- automatically register MSMAPI32.OCX on each computer -- add this functionality to your application

- switch from ActiveX control to MAPI API functions -- mapi32.dll is included in most Windows installations, no need to register it

- use a solution based on Winsock ActiveX control; in this case you don't need any email client installed; though again you have to check if this control installed

- use a solution based on Winsock API functions -- ws2_32.dll