Sometimes the Out of Office auto-responder can get confused and it won’t send replies correctly, or even at all. Sometimes it sends an old reply instead of the new one the person has entered.

You can delete these rules from the mailbox in Microsoft 365 via PowerShell with the following sequence of commands:

Connect-ExchangeOnline -ShowBanner:$false
Set-MailboxAutoReplyConfiguration -Identity [email protected] -AutoReplyState Disabled
Set-MailboxAutoReplyConfiguration -Identity [email protected] -AutoReplyState Enabled
Get-InboxRule -Mailbox [email protected] -IncludeHidden
Remove-InboxRule -mailbox [email protected] -Identity "Microsoft.Exchange.OOF.InternalSenders.Global"
Remove-InboxRule -mailbox [email protected] -Identity "Microsoft.Exchange.OOF.AllExternalSenders.Global"

There may be only one of the mailbox rules listed – not all mailboxes will have both. Just delete each one as appropriate.