Once I got an auto reply from a stakeholder on a project that said something to the effect of “Thanks for contacting me. Due to the large volume of email I receive, I don’t read them all. If I haven’t responded within 3 business days, please try again.”
An out-of-office message is an effective way to inform people why you are not responding to their calls or messages and when they can expect a response. This information saves them from getting frustrated and avoids any negative impression from the inability to respond. If you have long-term relationships with your customers, such an auto-response system is highly significant as it avoids any inconvenience.

I will be out of the office starting (Starting Date) through (End Date) returning(Date of Return). If you need immediate assistance during my absence, please contact (Contacts Name) at (Contacts Email Address). Otherwise, I will respond to your emails as soon as possible upon my return. What is the best out of office message?
I wrote the above comment off the top of my head. I wish I had time to rewrite and edit it. I would have changed “their goldfish” to “a spider they accidentally stepped on”, and would have added more detail to the story of the sister’s death (e.g. “her Pomeranian yapping” rather than the less descriptive “her dog barking”). Unfortunately, I could not do the thorough writing job required for that comment because someone close to me recently … – The person whose out of office advertised his gig on the weekend, for anyone in travelling to [city] – The people in a certain department who have taken to saying things like “if you really need to contact me, call 000-YYY-XXXX where Y is the square root of [insert numbers] and X is the year plutonium was discovered.” – The ones where people have an auto response saying they only check their emails once a day between 1-2pm – “I’m on research leave and I may be slow to reply.” (Whereby it is guaranteed they will reply immediately, because academics do not *really* take breaks).