Sent emails have disappeared - twice

Twice within the last 30 days, ALL of my "sent" emails have disappeared.  I have taken no action to delete those messages.  My Inbox seems unaffected, as all messages received that I wanted to keep are still there.  This situation is true no matter how I access my mail, and it makes no difference which device I use.  My spouse also has Cox email, and has suffered no such losses.  I have only used about 25% of my GB allowance.  Does anyone have any idea what is going on here?  The agents at Cox could not help me, but did suggest that I sign up for a service plan (they never stop selling, do they?).  I assure one and all that I am not deleting my sent messages while in a somnambulant state.  For now, I have started bccing myself on my sent emails.

Any suggestions are welcome here.  I have been a Cox customer for many years, but am thinking that perhaps a divorce from them is in order.