hi,

we are having couple of users both on MAC os and using outlook 2011 and 2016 and both has the same issue. we are using exchange 2013 cu8. 

when they send an email ( and its randomly no specific users) they can see the message in sent items folder but it shows as draft and never get delivered until they have to resend it again. issue not happening from OWA, but while the message stuck in the sent items folder as draft , its also shows as draft in OWA, checked autodiscover configuration all correct , did the connectivity analyzer it was successful. 

any ideas on what is the reason