The couple's flag gave the writer "the courage to come out," said the anonymous letter.
The note carried no signature, but it conveyed a mighty message.
"Hello," it began. "You don't know me, but my name is ... We're moving away today, but I wanted to thank you. Seeing a pride flag waving so proudly outside your house every day has given me the courage to come out to my family and be more comfortable with who I am."
Beneath was a cartoon drawing of a person holding a trans flag and a pansexual flag.
Sal Stow and her partner, Meghan Stabler, proudly fly their gay pride flags from a pole in their Texas front yard. Earlier this week, as Stow answered the door to receive packages from the postal carrier, she saw a sheet of paper anchored by a rock on the welcome mat.
She was flummoxed. "You can imagine how shocked I was," she told InsideEdition.com Friday. "I think speechless is the word."
Stow, who is a teacher and a gay rights and human rights advocate, studied the handwritten letter. The printing, she said, looked like it belonged "to a young person who's of middle high school age."
But she didn't want to speculate on who sent the missive, she said. "We really do not want to cause any harm to this person," she said of her and her partner.
They were particularly touched that the note arrived in the middle of LGBTQ Pride Month. Stow posted the letter to her Facebook page, as did Stabler. And of course, it went viral. Virtually every comment was positive.
"Thanks so much for sharing this. I immediately thought of Harvey Milk's entreaty 'Come out, come out, where you are,''' wrote one poster.
"Amazing!" wrote another. "We will (be) putting a permanent pride flag up soon. Inspired!"
The anonymous message was inspiring to Stow and Stabler as well. They live in a "very conservative county," Stow said. The two flags billowing in front of the home's door are visible to all. "Every time you drive by the house, you can see them."
After discovering the note, the first person she wanted to tell was her partner, Stow said. But Stabler was on an overnight flight to London. So she fired off a message, thinking, "I need to let Meghan know. I hope this person is OK and their family is being supportive."
The couple has been together for about 18 months. They met while working for the Human Rights Campaign, the country's largest LGBTQ advocacy group and political lobbying organization.
"We both believe in the same things," Stow said. Paramount among them, she said, is the importance of letting your pride flag fly.
Reading that stranger's letter hammered home that point. Encouraging someone to be themselves, and to step forward, "is why we need to fly our flag," she said. "That to me is the most precious gift of all — that other people are being inspired to be visible. That it will be OK."
Stow and Stabler had one more message to relay to the letter's writer.
"If that person does read this article, we want them to know we support you. We love you, and we are here. If they need to reach out to us, they can."