One user enabled his OOF but Exchange refused to send oof messages. Tested another mailbox – worked fine. I tried to check using Get-MailboxAutoReplyConfiguration and all seemed correct. Also checked that ExternalOOFOptions was enabled on his mailbox Get-Mailbox ALIAS | fl exter*  – result : ” ExternalOofOptions        : External”. Logged on to his mailbox using OWA checked that OOF was enabled and looked correct.

This user had a lot of rules so I increased his RulesQuota, also checked that he had not exceeded his mailbox quota. No luck 🙁

Finally copied his message to notepad. There was the problem. Some strange sympols in his Message. Formatted it as plain text in Notepad and pasted it back into OWA. Problem solved.