“Dear John letter” sounds a little like “Dijon letter.” “We can assume that when a mustard heiress throws over her boyfriend she sends him a Dijon letter” was printed in the Doug Larson column in the Green Bay (WI) Press-Gazette on August 13, 1986.
 
A riddle form was posted on the newsgroup alt.callahans on March 25, 2000.
 
Did you hear about the man whose wife ran off with a mustard salesman?
She wrote him a Dijon letter. (Cathleen Shoemaker)
   
     
Wikipedia: Dear John letter
A Dear John letter is a letter written to a man by his wife or romantic partner to inform him their relationship is over because she has found another lover. The man is often a soldier stationed overseas, although the letter may be used in other ways, including being left for him to discover when he returns from work to an emptied house.
 
13 August 1986, Green Bay (WI) Press-Gazette, Doug Larson column, pg. A-15, col. 5:
We can assume that when a mustard heiress throws over her boyfriend she sends him a Dijon letter.
   
18 January 1987, Paducah (KY) Sun, “Senator Soaper,” pg. C3, col. 5:
We can assume that when a mustard heiress throws over her boyfriend, she sends him a Dijon letter.
 
9 April 1991, Milwaukee (WI) Sentinel, “New breed of dog has good ear” by Alex Thien, pg. 3-1:
DALE RADKE says the young lady sent her boyfriend a letter telling him that he couldn’t cut the mustard.
 
After reading the letter a number of times, he decided it was a Dijon letter.
     
Google Groups: rec.humor
Life 9.P
Henry CateIII
9/9/94
(...)
Hear about the man whose wife ran off with a mustard salesman?
She wrote him a Dijon letter.
 
Google Groups: alt.callahans
Puns of the Weak: 3-24-00
Stan Kegel
3/25/00
(...)
Did you hear about the man whose wife ran off with a mustard salesman?
She wrote him a Dijon letter. (Cathleen Shoemaker)
     
Twitter
Coy limbo
@MicLooby
Did you hear about the woman who left her husband for a jar of French mustard? She sent him a Dijon letter. Wacka wacka wacka.
12:31 AM - 22 Jan 2010
   
Twitter
Laurence Rickard
@Lazbotron
All of my condiments have walked out on me. I woke up to find a Dijon letter.
4:00 AM - 31 Jan 2010
   
Twitter
Dave Bromage
@davebromage
A girlfriend once dumped me with a note written in mustard. It was a Dijon letter.
2:25 AM - 8 Apr 2010
 
Twitter
Peter Smith
@Redpeter99
My girlfriend just ended our relationship due to my mustard addiction. Got a Dijon letter today. #UKPunDay
7:35 AM - 12 Feb 2018
 
Twitter
🤣 The Dad Joke Man 😉
@DadJokeMan
I used to be so obsessed with mustard, my wife couldn’t take it any longer.
So one day she packed her bags, and left me a Dijon letter…      #TuesdayTitters
1:43 AM - 16 Oct 2018