You are getting a Delivery Status Notification about your Vacation Message similar to this:
From: Mail Delivery Subsystem <firstname.lastname@example.org>
Date: Wed, Jul 4, 2018 at 3:21 PM
Subject: Delivery Status Notification (Failure)
Message not delivered
There was a problem delivering your message to KronosPROD@ucsb.kronos.net. See the technical details below. LEARN MORE The response was:
The recipient server did not accept our requests to connect. Learn more at https://support.google.com/mail/answer/7720 [ucsb.kronos.net 126.96.36.199: timed out]
Final-Recipient: rfc822; KronosPROD@ucsb.kronos.net
Diagnostic-Code: smtp; The recipient server did not accept our requests to connect. Learn more at https://support.google.com/mail/answer/7720
[ucsb.kronos.net 188.8.131.52: timed out]
Last-Attempt-Date: Wed, 04 Jul 2018 15:21:28 -0700 (PDT)
---------- Forwarded message ----------
From: person <email@example.com>
Date: Sun, 1 Jul 2018 11:05:15 -0700
Subject: Out of the office Re: Timecard Approval Reminder
Thank you for your message. I am away from the office and will return on Tuesday, July 3.
You received the message even though your Vacation response should have been off.
This is a normal behavior in situations such as the recipient is not accepting messages, such as this automated message from KronosProd.
When you look at the contents of the message, it says:
The recipient server did not accept our requests to connect. Learn more at https://support.google.com/mail/answer/7720
Which means there were several attempts to send this message to the recipient.
Also, look at the Forwarded message section.
The date in which the message sent to the recipient was on July 1st, 2018.
The person's Vacation message was enabled around June 27th and did not end until July 3rd.
The response was sent out during the vacation message time.
The mail system will try to deliver a message for several days before it finally gives up, which can happen after your vacation message deactivates.