Denver Broncos quarterback Peyton Manning said an emotional goodbye to the National Football League March 7, after an 18-year career that established him as one of the game's greatest quarterbacks. (Reuters)

The letter came in the mail, and London Fletcher separated it from the pile and carried it into his home office.

The stationery was distinctive, the name on the return address unmistakable: Peyton Manning. Fletcher, the undersized but overachieving Washington Redskins linebacker, had — after 16 NFL seasons and 215 consecutive starts — announced his retirement shortly after the 2013 season. The following spring, he opened the envelope and began reading.

Congratulations, Manning had handwritten on a single page, on a career well spent, for respecting the game, for leaving such an impression that Manning’s Denver Broncos had named a play for Fletcher’s jersey No. 59.

“One of the classiest things,” the former linebacker said Monday in a telephone interview, “that I’ve ever had displayed to me.”

Fletcher didn’t realize it at the time, but he had joined an exclusive club. Long before Manning himself announced his retirement Monday during an emotional news conference in Denver — “God bless football,” he told reporters — the legendary quarterback spent offseasons making a list of players he had observed and admired. He often wrote to them, usually after they retired or reached a milestone, and scrawled his own memories and best wishes, usually taking note of each recipient’s respect of a game Manning spent decades attempting to master.

Fletcher received one, and so did former defenders Troy Polamalu and Ed Reed. Those players had tormented Manning, and in a strange way, the quarterback appreciated it. Donald Driver, the longtime Green Bay Packers wide receiver, said in an email Monday that he wondered if the handwriting in his own note could really be Manning’s. He found a Colts helmet Manning had signed for Driver at the Pro Bowl and then compared the signature with the one on the letter and, yes, it was legitimate. Shannon Sharpe, the former tight end, wondered how — and, perhaps more revealingly, why — Manning had spent the time writing to Sharpe, then tracking down the address of a player Manning had met no more than a handful of times.

“What a tremendous honor and entirely well-deserved,” the quarterback wrote to Sharpe in March 2011, shortly after Sharpe was selected for entry in the Pro Football Hall of Fame.

“I was like: How did he get my address?” Sharpe said Monday. “He thought enough that, I could have my secretary type up something — but that wouldn’t do it justice. To sit down and to put pen to paper, see, that takes time.”

Manning seemed to take particular interest in players who studied the game, sidestepping opponents’ increased attention and finding a way to evolve, survive and succeed in an unforgiving league. Manning, a legendary student of the game whose physical skills had clearly begun deteriorating long before Denver won this past season’s Super Bowl, had reinvented his own approach in recent years, forming an acute appreciation for the players who continued thriving in an unforgiving league.

“It’s one thing to have talent,” Fletcher said. “But it’s another thing to have talent and work ethic and preparation.”

Years earlier, Manning told the Los Angeles Times in 2013, he learned to appreciate the power of a handwritten note. His mother, Olivia, encouraged her three sons to send thank-you cards and to write letters. Manning remembered, according to the Times article, being a teenaged phenom closely inspecting a recruiting letter from former Florida State coach Bobby Bowden, smearing the coach’s signature to make certain the ink was real.

“Boy, it had a big impact on me,” Manning was quoted as saying by the Times. “He took the time to write that letter.”

Manning, even in an age when letters and calls have been replaced by texts, kept writing. He asked his teams’ public relations staffs to contact other franchises to track down other players’ mailing addresses. Sometimes the return address was from Manning’s charitable foundation; other times the notes were sent with the Colts’ or Broncos’ outgoing mail.

He planned in 2013 to write to former offensive lineman Matt Birk and, the Times reported, sent a case of wine along with a note to former Broncos safety John Lynch.

Fletcher said he noticed that Manning had his own stationery, the former linebacker wondering why he had never sprung for such a luxury. Sharpe appreciated the time and sentiment of the pen-to-paper approach, trying to remember the last time he sat and crafted an honest-to-goodness letter. “Thirty years,” Sharpe estimated. “Probably a love letter.”

This past weekend, Manning took a more modern approach. Sharpe, who had Manning’s note framed, said he received a series of text messages from Manning on Sunday morning. The pair had grown closer since Manning joined the Broncos in 2012, and Manning wanted to tell Sharpe — before the news became public — that now it was his turn to walk away.

Sharpe responded, applauding his friend in a text on a legendary career and wishing him a happy retirement. Fletcher, anyway, wondered if something more formal might be more appropriate. Maybe he would get around to ordering that stationery, finally, and sit in his home office and write a note to Manning.

Fletcher wouldn’t say Monday what he might tell the now-former quarterback, saying the message should remain between writer and recipient. But he figured he’d start it the same way his own note began: with congratulations, something as simple as that, on a career well spent.