Outlook re-open to send mails

friends i am sure this is not Outlook question so posting it here.

i have Exchange 2013 and generally i keep my outlook open.. but when sending mails it stuck in outbox and i have to close and reopen the outlook for the mail to go out. as if outlook looses the connection. i have outlook 2013 all patched.

not sure what to check.. not i am sure that this doesnt happen becoz of my outlook being open for long as at times i start app.. create mail and send it but it stucks in outbox and same re-open the app and then it goes.

please share your ideas what can be checked. this is happening with more then one user thats why i think this has something to do with exchange.