[Free dope, too: a couple factoids re ISP PROBLEMS setting up mail for these late OS X builds + the fact of a malware test, made on existing s/w BEFORE they commence new install.]

A confluence of techno-grade-nastiness caused me to call [they actually Did call-back in seconds!] the Apple 'Pay'-number! With trepidation.
Amidst weirdness, slow operation, general need to reboot at least every couple weeks--usually via manual shutdown--as both Mail and Safari would not obey 'Force Quit' (and other details likely more boring than illuminating) I decided to call the Pay-number (!)

(Just to make it an interesting Glitch: amidst above,) Time Machine HD (tiny WD Passport drive) rock-solid: Disk Utility came up in Red/it's broke.
Ran Verify, thence est. "a couple hours"==400 GB drive. Then said, it's OK now. But TMachine indicated.. "what drive you talkin 'bout, Willis?
Back to Verify/2 more hours then "OK" again. This time TM indicated "preparing" or like that. And it worked. {sigh} shitty timing, that.

So then: Apple guy, after Q &A about it all and my query: would it be simpler to kiss off the analysis/any deep sleuthing and just upgrade to Sierra? (was on Yosemite.) At this point he mentioned **[see below the line] and also informed that an early '09 iMac-24 could only run El Capitan/a late '09: can do Sierra. He concurred that this (last for my hdwre) upgrade was simplest solution.

Jeez I Hate the double-security 6-digit confirmation codes, but at least THIS OS X install had, at that appropriate screen, "Don't have a security code?: A) here's how, on web
OR door-B) "you can CALL us and we'll call back with it!" real-Soon! I did and in seconds They Did. Bitchin, that.

tl;dr so the install was uneventful and the sucker flies, something it Hasn't for some months. *BUT* The New! email ap wasn't talking to my ISP, no response from GetMail button nor my TEST message.

Now, Rob at my ISP I've known for the ~20 years beginning with dial-up. (They also did test equip. in early days.)
He mentioned the assholery of (dunno how many Animals-past) of these post-Animal offerings: his having to easter-egg all around to configure them,
that it was costing them $thousands ... at a half-hour typ. way too frequently.
(Seems that Apple has yet to see the wisdom of circulation to the 8000 U.S. ISPs: **a manual for the insanely-great-new changes:
largely hiding beneath new drill-down menus,replacing TRADITIONAL locations for configs.

POINT: he's considering dropping 'just-MAIL' (my current use of his ISPery) as I have to use Comcast in my tiny village because [never mind: tmi.
Hope not, as they have been Gold since day1 and I know not where else someone will take me in for Mail only, at a pittance.

En fin: this was on a Sunday; we spent near 40 minutes (some asides of irrelevancies) and NO CHARGE.

fortunately though, never used the Comcast e-mail addy auto-assigned: made clear to them that my e-mail addy is not Theirs, should some e-mail seem necessary.
(Amidst whatever $$ or other chaos has been unleashed, I phone the suckers.)

Heh, their latest ad-blitz seen on PBS, after the xfinity cha. cha. is: the claimed hiring (over n-years, in tiny print) of ex-military folk, the implication being

A) We'll be here forever/ and we're patriotic as all get-out.
B) Guess we're supposed to believe it's because of new business/not re perennial sloppy service, heretofore.

But who's believing either one (?)