I suggest that instead of sending an email, why don't you organize a high tea with all the employees who have completed one or two years separately, along with your COO/CEO or unit/area head? This approach makes a lot of sense rather than relying solely on your e-savvy. Please discuss this with all your HODs, finalize the date/time/venue, send out invitations, organize the event, and offer congratulations. Additionally, you can identify high performers and acknowledge their contributions.
Regards,
Kamesh