Mail Merge for letters or emails
If you are using Outlook to send a similar email template to a few different people then the simplest way to do that is to just make your template in an additional extra long Outlook Signature (see Office Support instructions of see also this video)
MS Word has a mail merge feature that can be used with other office products like Outlook, Excel and Access to create merge your contact list into a mailing or bulk email. When you use the merge feature then each mailing or email looks like it was sent to one recipient.
But for longer mailing lists people often use MS Word to create a mail merge