Advice for automated tax acknowledgement letters

I thought I could search for a way to trigger tax acknowledgements via email but I can't seem to find such a thread...

• What segment criteria you use?
• What is your contribution minimum amount, if any?
• Is this through List Manager or Extraction Manager?

Currently we send in the mail for donations $25 and up but want to lower the threshold and move to email-automation. Would love to hear how it's done!