The North: grin and Bear it
Few Game of Thrones characters have been introduced as gloriously as Lyanna Mormont. The no-nonsense 10-year-old ruler of Bear Island will see you now -- and she will also stop you cold mid-sentence should you make any attempt at chitchat. "Get to the Point" are the unofficial words of House Mormont from here on out. On a per-second screentime basis, Lyanna's badass quotient is challenged only by that of Smalljon Umber, who memorably razzed Ramsay Bolton earlier this season. But Lyanna wipes the floor with that guy, too, because Lyanna is undoubtedly the greatest character in the history of Game of Thrones.
Jon and Sansa learn all too quickly that you can't bring weak stuff to Lyanna. As the Stark kids dance hideously for Mormont table scraps, only to be bailed out by the wisdom of Davos, they're served all of the following rejoinders by the littlest Mormont (who happens to be Jeor Mormont's niece and Jorah Mormont's cousin):
"I think we've had enough small talk. Why are you here?"
"My mother was no great beauty, or any other kind of beauty."
"If you say so. In any case, you don't just want my allegiance -- you want my fighting men."
"I understand that I'm responsible for Bear Island and all who live here. So why should I sacrifice one more Mormont life for someone else's war?"
The indignities Jon and Sansa suffered on Bear Island to net a measly 62 Mormont fighting men! Long live Lyanna Mormont (who will probably die soon, because it's Game of Thrones).
Jon, meanwhile, seems to be unusually dead inside. While grimly beseeching the wildlings to join in their cause to take back Winterfell, he looks like he's about to cry. This may be because the wildlings are looking more and more like extras from the "Dancing With Myself" video, or he may be bummed out about shit getting into his stab wounds, or it might be because he no longer thinks he has a righteous cause on his side. Jon is (rightfully) more concerned with the danger lying to the North than the political morass he's being sucked into in the South, and he's Mr. Gloomy Gus about the enormity of it all. Whatever the reason, Sansa's not helping to cheer him up.
I'll say it: what the hell is Sansa doing? First, she reminds a borderline hostile Lord Glover of his obligation to rally behind the Starks, a poor decision that causes Jon to sigh and Lord Glover to refer to Robb Stark's dead wife as a "foreign whore." Then Sansa disses Davos snarkily to Jon's face after the Onion Knight had single-handedly won Lyanna Mormont over to their cause. Finally, Sansa schemes. Without telling Jon or anyone else of her plans, she fires off (quills out?) a letter, destination unknown, after being seemingly upset with Jon's insistence that they march on Winterfell as-is, with only 2,000 wildlings, 200 Hornwoods, 143 Mazins, and 62 Mormonts. No one puts Sansa in the corner.
So whom did she send the letter to, via Mormont raven? It can't be going to any of the Northern houses already mentioned, including Houses Glover, Mormont, Manderly, Mazin, Hornwood, and Cerwyn. It can't be the Blackfish, because Sansa has already dispatched Brienne to Riverrun to enlist her great uncle and his men in their fight. Did she send it to Littlefinger? Possibly -- but the letter was signed a bit too formally, not to mention that asking for his help would undermine everything she said to him during their showdown earlier this season.
My three guesses for where the raven is heading: to her cousin Robert Arryn, aka Sweetrobin, who is partial to her vs. Littlefinger and who is presumably still holed up at the Eyrie; to Yohn Royce, who hates Littlefinger with a passion, who was a regular hunting partner of Ned Stark, and who is conveniently camped out with the Knights of the Vale south of Winterfell at Moat Cailin; or to Howland Reed.
My hope is that Sansa sent the raven to Howland, father of Jojen and Meera Reed. He's among the loyalest Stark bannermen who, remarkably, has gone unheard from for the entire series (in the books and on the show), save for helping Ned beat Ser Arthur Dayne in the flashback Bran had at the Tower of Joy. It's unclear how many men Howland Reed commands, but his surfacing on the side of the Starks would no doubt draw any noncommital Northmen toward the Stark cause.
But yeah, she probably just sent it off to Littlefinger.