Sometimes it is necessary to use powershell commands to enable an out of office reply/auto reply message for a mailbox.
You must connect to office 365 via powershell first. To connect to office 365 via powershell follow this guide.
Check the mailboxes out of office (OOH) reply status:
Get-MailboxAutoReplyConfiguration -Identity [email protected]
Turn on the mailbox out of office (OOH) reply:
Set-MailboxAutoReplyConfiguration -Identity [email protected] -AutoReplyState Enabled -InternalMessage "This is the message to sent internal users" -ExternalMessage "This is the message sent to external users"