Is there an app or process that produces bulk mailings of individualized HTML emails?

For almost 2 years, my graphics work has been donated to a local nonprofit. They mostly use Microsoft Office apps. I have that plus the current Adobe CC Suite. They would like to compile a list of individuals that a bulk mailing would send nicely-styled HTML emails (with the message in the email itself, not attached to it). The emails would be individualized in the Send To field and would include the recipient's name and perhaps other specifics in the body of the email.

Past e-mailings of theirs have used PDF attachments to hold the message. They weren't successful.

IOW the grouped emails should look like each one was sent by the nonprofit to a single recipient, including the info relevant to that recipient. E.g., how long the recipient has supported the nonprofit.

I know nothing about how that would be done, but am definitely willing to learn.

Chuck M