We don't know who they were sent to but can probably speculate at a couple of options.
Let's have a look at what the letter said to see if we can work out who he would send them too.
"...not the only Targaryen left. Rhaegar and Lyanna... their son lives still, hidden by Eddard Stark... he is the true heir to the Iron Throne..."
Game of Thrones, Season 8 Episode 5, "The Bells" - Varys' letter
So of course he's telling people that Jon is in fact the true heir to rally support for him over Dany. So to work it out we need to work out who are the best people to find out Jon's heritage to turn to his side. Well back in Season 2 Stannis sends out letters to all the lords in Westeros telling them of Joffrey's parentage.
Matthos: And I declare upon the honor of my house that my beloved brother Robert le... That my brother Robert left no trueborn heirs; the boy Joffrey, the boy Tommen, and the girl Myrcella, being born of incest between Cersei Lannister and her brother Jaime Lannister. By right of birth -
Stannis: Jaime Lannister, the Kingslayer. Call him what he is.
Matthos: And her brother Jaime Lannister, the Kingslayer. By right of birth and blood, I do this day lay claim -
Stannis: Make it Ser Jaime Lannister, the Kingslayer. Whatever else he is, the man's still a knight.
Matthos: Ser Jaime Lannister, the Kingslayer. By right of birth and blood, I do this day lay claim to the Iron Throne of Westeros. Let all true men declare their loyalty.
Stannis: When Eddard Stark learned the truth, he told only me. I'll not make the same mistake. Send copies of that letter to every corner of the Realm, from the Arbor to the Wall. The time has come to choose. Let no man claim ignorance as an excuse.
Game of Thrones, Season 2 Episode 1, "The North Remembers"
Varys is almost certainly doing the same thing here. So what lords do we know of that don't know this information and probably would want to? Well the only two I can think of right now that have the most importance would be the new Prince of Dorne and Yara Greyjoy, there would of course be others but they're the only named ones. And we might add in the Reeds and wildlings there too but they seem to have left the conflict altogether now.
Gendry Baratheon was also missing from the battle and as a newly legitimised bastard who is made Lord of Storm's End he would most certainly be on Dany's side initially. However, given the news he might also turn to Jon's side and rally support for his case, especially for his feelings for Arya.