EMAILS DISAPPEARING FROM SENT ITEMS - NO IDEA IF THEY'VE SENT

EMAILS DISAPPEARING FROM SENT ITEMS - NO IDEA IF THEY'VE SENT

Hi 

I just sent a hugely important work email - a green tick appeared when i pressed send but it is not in any of my folders, sent , deleted, archived , outbox etc.

Please fix this and contact me ASAP this is unacceptable and the 2nd time it has happened recently! 

cheers
nerissa