What actually happens behind the scenes
Once enabled, an automatic reply typically triggers only once per sender within a set period (usually a few days), rather than firing on every single message from the same person -- which is why you may not see a reply sent for every follow-up email someone sends you while you're away.
Internal vs. external messages
Outlook, in particular, lets you write a more detailed message for people inside your own organization and a shorter, more general one for outside senders, which is useful if you want to share more specific coverage details internally without exposing them externally.
Frequently Asked Questions
Will an automatic reply go out for every single email I receive while away?
No -- most systems limit automatic replies to once per sender within the set period, to avoid repeatedly auto-replying to the same person or getting caught in a reply loop with another automated system.
Can I turn on an automatic reply from my phone?
Yes, both the Gmail and Outlook mobile apps let you enable and edit automatic replies from the same settings menu as the desktop version.