It is a quotation from Episode Three of The Hitchhiker's Guide to the Galaxy radio series by Douglas Adams.
This is a comedy, the phrase, as used there, is to signify that Dolphins are more intelligent than humans.
In the story, Earth is destroyed, the dolphins knew this was coming and left the planet.
The full quotation is: Curiously enough, the dolphins had long known of the impending demolition of Earth and had made many attempts to alert mankind to the danger.
But most of their communications were misinterpreted as amusing attempts to punch footballs, or whistle for titbits, so they eventually gave up and left the Earth by their own means - shortly before the Vogons arrived.
The last ever dolphin message was misinterpreted as a surprisingly sophisticated attempt to do a double backwards somersault through a hoop, whilst whistling the ‘Star-Spangled Banner’.
But, in fact, the message was this “So long and thanks for all the fish”.
Subsequently, Douglas Adams published a book with the title
So Long and Thanks for all the Fish which was based upon the original series.
So, in the context of your email, it is just an attempt at humour by someone leaving.
ell.stackexchange.com/users/4376/Chenmunka
To boil it down to its most basic: It's a quote from The Hitchhiker's Guide to the Galaxy and has become a humorous way of saying "Goodbye".
ell.stackexchange.com/users/27247/Tobey
"So long and thanks for all the fish" is the title of the fourth book from the "Hitchhiker's Guide to the Galaxy" tetralogy.
Used in a message it is just a humourous way to say goodbye, calling to mind the leaving of the dolphins from Earth saying thanks for the fish they had gotten.
ell.stackexchange.com/users/32933/Victor B.
As the others have said, the line is a quote.
The phrase "So long" is an informal synonym for "Goodbye".
ell.stackexchange.com/users/46170/Hobbes
The line is the title of a humorous novel by Douglas Adams.
It refers to dolphins, who were much more advanced than we had thought, leaving Earth prior to its destruction to make room for a "hyperspace bypass".
Having a particularly British sense of decorum, they could not leave without thanking us for the fish they had eaten.
I have no idea what your manager's email was about.
In context with its subject line, it could be that he is good-naturedly announcing he will be away from the office for a while, or maybe permanently.
ell.stackexchange.com/users/46961/RichF
I just recently used this expression, and got the same question.
Other answers (especially the one from Chenmunka) have described the reference, but I’m not sure they fully describe what I was trying to say.
For me, and I believe most others, it's a humorous way to say 3 things simultaneously: Goodbye.
Thanks for all the nice things you’ve done for me.
The goodbye is permanent: I’m not coming back, and most likely I’llnever see or hear from any of you again.
It’s that third connotation that makes people want to use humor.
It keeps the goodbye from being too melodramatic.
ell.stackexchange.com/users/41432/Richter65
As others have said - it is a quote from Douglas Adams the Hitchikers Guide to the Galaxy.
Almost certainly he is referencing saying goodbye - probably on a permanent basis or possibly simply for a long time.
ell.stackexchange.com/users/47964/kiltannen
Adams' line "thanks for all the fish" might be a nod toward the intertextuality of several science fiction writers.
Vonnegut's Kilgore Trout being his nod towards his fellow writer Theodore Sturgeon, Philip Farmer's "Venus on the Half-Shell" under the pseudonym Kilgore Trout nods toward Vonnegut, and then Adam's Hitchhiker's Guide to the Galaxy" nods toward 'Half-Shell's' 'definitive answer.
And one more nod
perhaps: half-shells come from oysters, a type of shellfish anyhow.
ell.stackexchange.com/users/92351/Bonobo
I too used the phrase when departing a disagreeable job.
I held a very negative outlook for the companies future, ie.
Imminent failure.
I was happy to leave for a better job, yet felt sorry for the poor souls trapped in their jobs.
I gave proper notice, left quickly and had no further contact with anyone after my departure email....
its been nice working with you....
blah blah blah....
so long and thanks for all the fish.
The quote and story line seemed to fit my job experience
so bloody
well -on
so many levels!
I enjoyed eating at expensive restaurants using my expense account
and I'm a pescatarian!
I thought the intent of the quote (a humourous middle finger) in my departure email was so obvious.
Now I think people had no clue how negative I was about the company's.
ell.stackexchange.com/users/107121/Sarcasticat
I got a mail with the subject  "So long and thanks for all the fish!"  from my Manager. What does it mean?
What does the line "So long and thanks for all the fish!" mean?
ell.stackexchange.com
<meaning-in-context>
ell.stackexchange.com/questions/115782
ell.stackexchange.com/users/47645/Chaitanya Kumar
It is a quotation from The Hitchhiker's Guide to the Galaxy, by Douglas Adams.  It has become a humorous way of saying "Goodbye". So, in the context of your email, it is just an attempt at humour by someone leaving. This is a quote from Douglas Adams' "The Hitchhiker's Guide to the Galaxy". In the original comedy, the phrase inferred that dolphins are smarter than humans. In this context, the manager wanted to say "Goodbye", probably forever or for a long period of time, in a humorous way.
This is a quote from Douglas Adams' "The Hitchhiker's Guide to the Galaxy". In this context, the manager wanted to say "Goodbye", probably forever or for a long period of time, in a humorous way. In the original comedy, the phrase inferred that dolphins are smarter than humans.