Neither rain nor sleet nor passage of nearly a half-century could stop this postal delivery.

A Brooklyn woman was shocked when she received a weathered, kiss-sealed letter last week that her mother sent for her 19th birthday — in 1969!

The wayward letter arrived at the childhood home of Susan Heifetz in Sheepshead Bay on Wednesday, prompting the baffled tenant to seek out the intended recipient.

The man found a phone number online for Heifetz, who works as a real-estate agent, and gave her a call.

“I was like, ‘Oh, my gosh,’ ” Heifetz said, referring to her amazement when she was told about the smooched envelope.

“He said, ‘I have a letter for you and the only reason I’m trying to find you is because it’s postmarked 1969,’ ” she added.

Heifetz said her late mom commonly sealed letters with a kiss before mailing them, so when she dropped in on the good Samaritan Friday at the East 12th Street building where she grew up, she immediately recognized her mother’s lipstick on the back of the envelope.

“I was very emotional about it,” she said. “Her Max Factor lipstick 45 years later . . . it hasn’t faded.”

Inside was a tender birthday card dated June 27, 1969, wishing her health, happiness, success and a long life.

“Dear daughter Susan,” her mother wrote at the top of the card. “Mazel tov!”

The bottom was signed, “Love and kisses, Mamma Molly and Daddy Sam.”

Heifetz said she has been planning to move to Las Vegas, but has had some reservations about leaving her New York roots.

“I felt like this was a stamp of approval,” she said. “Like, [she was telling me] ‘We’ll always be in your hearts and soul. We found you 45 years later.’ ”

Mysteriously, Heifetz received two other letters — one another birthday card from her still-living brother. The other was penned by an old boyfriend who served in Vietnam.

“Remember me?” the letter, postmarked Oct. 25, 1969, began.

She remembered.

“He had tried to reach me before he went overseas,” Heifetz said. “But in 1969 there were no answering machines.”