The letters are going to addresses whose waste is sent to Associated Waste Management in Shipley where the baby girl's body was found on 19 May.
The site received 97 loads of waste the day before, 51 from Bradford and the rest from elsewhere in the county.
Police hope an appeal by letter may jog the memory of a potential witness.
The deliveries will be made to addresses in more than 2,000 post code areas, while a second letter will be sent to businesses to appeal to staff for information about the case.
Inquiries have so far revealed the baby was a newly-born white girl who died two or three days before her body was discovered by staff at the Valley Road site.
A West Yorkshire Police spokesman said: "It is also believed the baby did not have a hospital birth and locating the mother to make sure she is safe and well remains a priority for officers."
A team of detectives continues to work on the investigation and police have also been carrying out forensic work to examine other possible ways to identify the baby girl.
Det Supt Sukhbir Singh, who is leading the investigation, said: "The answer to finding the mother of this child continues to lie within the communities of West Yorkshire and we are carrying out this new information seek to see if we can uncover new information to help us find her.
"Any case which involves the death of a baby is harrowing and we are determined to explore all avenues to both identify this baby girl and locate her family."

The Hollywood star has set up his own call centre in the city's Fact gallery, where he and his two artistic collaborators are fielding calls.
They will be at their desks between 11:00 and 18:00 GMT from Thursday to Sunday.
Those wishing to touch LaBeouf's soul can call the trio on 0151 808 0771.
Others can visit the gallery to see the event unfold in person, or can watch a live stream and see notes the trio are making on Touchmysoul.net.
LaBeouf made his name in films like the Transformers series, Wall Street: Money Never Sleeps and Indiana Jones and the Kingdom of the Crystal Skull.
In the last two years, he has become known for art performances that examine the nature of fame and the gap between a celebrity's public persona and real life.
He wore a paper bag with the words "I am not famous any more" over his head on the Berlin Film Festival red carpet, set up a live stream for people to watch him watch all his movies one after another, and sat silently in a US gallery where visitors could come and say or do what they wanted to him.
He also gave a memorable motivational speech urging people to "just do it!", which became a viral hit.
This is his first such performance in the UK. He works with Finnish artist Nastja Sade Ronkko and Briton Luke Turner under the name LaBeouf, Ronkko & Turner.
Some critics dismiss their activities as stunts, but the actor told The Guardian they arise out of "an animalistic urge to express love that I can't express in film".
In Liverpool, the trio have been answering calls with the words: "You're through to LaBeouf, Ronkko & Turner, can you touch my soul?"
They are transcribing what the callers then tell them. According to their notes, they have been told:
The trio are part of a group exhibition titled Follow, which examines how connected people really are in the digital age and opens to the public at Fact on Friday.
Curator Amy Jones said: "They're definitely exploring the idea that we're more connected than ever but the challenge of making sure those connections are real and meaningful is harder than ever.
"This idea of people from all over the world being able to call up and really talk to another person on the other end is quite liberating.
"But also the idea of it being through a phone line - it's mediated, so there's a distance, so it's a challenge to overcome that distance, which is something we all experience."
Police are sending letters to 37,000 homes in West Yorkshire from this week in a bid to find the mother of a baby who was found dead at a waste site.
[NEXT_CONCEPT]
Actor Shia LaBeouf has launched his latest performance artwork in Liverpool, asking members of the public to phone him and "touch his soul".