It had been my understanding for awhile now (:p) that an adverb, when used in a compound adjective, does not require hyphenation. But MS Word blue-lined my well received memo.
Is it merely adverbs ending in -ly that are sans hyphen when used in a compound adjective?
(I’m not certain if sans requires italics, but it appears not, due to familiarity, despite it being a foreign-language word.)
Note: my apologies for the political incorrectness of the thread title.
Yes, well-received needs a hyphen.
Chicago Manual of Style, at least, says you should, unless the modifier comes after the noun (as in, “the man is well dressed.”) Cite. (Third page, under "adverbs not ending in ly).
Here’s another run-down of style points. Some style guides say that it doesn’t matter where in the sentence the compound adjective is (so those would have “the man is well-dressed.”)
So, as usual, follow your preferred style guide.
Usually you would hyphenate before the noun but not after.
“A well-received memo”
“A memo that was well received”
“A well-known celebrity”
A celebrity that is well known"
Hyphenation seems to show signs lately that some cases are going out of style, but your example needs a hyphen as far as I can tell.
It would be a well-hyphenated use. 
Somebody really needs to tell this to ZZ Top.
And do be careful about commas as well. “The well-dressed man” might not be too pleased if he read a description of himself as “The man was, well, dressed”!
Maybe they were referring to his acuity and clothed state, as a contrast to a “dull naked man.” 
Quotation marks are frequently misused as well. The “well-dressed” man might not be impressed with that description either.
Or he was dressed in a cape made from signs to raise notes by a semitone.
I assume the purpose of the hyphen is to make it obvious that the “well” is an adverb modifying “received” rather than an adjective modifying “memo.”
One of the examples in the link is “the best-known author.” Without the hyphen (“the best known author”), it’s ambiguous: Is it the best author who is known, or the author who is best known?
In some cases, such as the “well-received memo” example, there’s only one interpretation that really makes sense, but still the hyphen saves the reader from having to figure that out for herself.
Go, thou, and read you Chicago manual of Style! If dull were to apply to “man” it needs to be separated by a comma, as in dull, naked man. Ha!
I was waiting for someone to point that out. 
Exactly right. This is why some of us appear to be prescriptivists some of the time, yet in an apparent inconsistency are oddly accepting of many non-standard usage variations. It’s just that some language rules have a real functional utility, and that applies even to the function of alleviating a little bit of extra mental processing rather than actual irresolvable ambiguity.
And yet, Miss Thistlebottom still gets all strung out by my misplaced modifiers.
It’s one of her hobgoblins.
How would one punctuate an “extremely well received” memo?
Serious question. I really don’t know.
Use the must-hyphenate-all-words-in-a-long-compound-adjective rule.
Adverbs ending in -ly, though, do not get a hyphen, at least according to the manuals of style I’m familiar with. So the question is between “extremely well-received memo” and “extremely well received memo.” I would go with the former. The Chicago Manual of Style says: “When the adverb rather than the compound as a whole is modified by another adverb, the entire expression is open.” So is “extremely” modifying “well” or “well-received”? If the former, then it’s “extremely well received memo.” Hmm… So now, I’m actually leaning towards the open version. ETA: Or now I’m thinking to go with my initial thought with “extremely well-received memo.” That just feels “right” to me, but it’s just an issue of style.