City Hall withheld nearly two dozen emails that show Mayor Bill de Blasio had a much cozier relationship with Jona Rechnitz than he has admitted — even telling the crooked developer to reach out “anytime I can help,” The Post has learned.

De Blasio was corresponding with Rechnitz for more than two years, from late in his first campaign in October 2013 through February 2016, despite the mayor’s April 2016 claim that he and the admitted briber were “not particularly close,” the previously undisclosed emails show.

“Jona, really enjoyed our mtg,” de Blasio wrote on Oct. 4, 2013, following receipt of $36,700 in cash Rech­nitz had collected for him. “Call upon me anytime I can help. And thanks for your extraordinary assistance for my cause — means a lot to me.”

Other emails obtained by The Post show de Blasio calling Rechnitz “my friend,” “brother” and “a mensch” — Yiddish for “a person of integrity and honor.” One even reveals that he told Rechnitz to keep their communications discreet.

“And always stay in touch, but please do via this email or via cell rather than text,” de Blasio wrote from his personal email account — deblasio@att.blackberry.net — on Oct. 2, 2014. “Much easier for me. Thanks again for all your help.”

In addition to the terms of endearment that de Blasio heaped on Rechnitz — who cut a cooperation deal with the feds to avoid getting busted with pal Jeremy Reichberg in a hooker-fueled NYPD corruption case — the never-before-seen emails show the mayor repeatedly thanking Rechnitz for advice and information on which de Blasio later acted.

The correspondence also reveals the usually thin-skinned mayor graciously accepting an apology from Rechnitz for an unexplained slight, writing, “No worries” during an exchange of emails on Dec. 3, 2013.

And when Rechnitz, now 36, wrote, “Thnx I only mean well and am young and learning,” de Blasio, 57, wrote back two minutes later, taking a fatherly tone as he said, “We are all learning . . .”

The 21 emails written by de Blasio were not produced by City Hall in response to a 2016 Freedom of Information Law request by The Post, even though officials turned over 286 pages of correspondence involving de Blasio, Rechnitz and Reichberg during 2014.

The newly discovered emails between de Blasio and Rechnitz also add to others that prosecutors unveiled last month in Manhattan federal court, where they were introduced as evidence against Reichberg and former NYPD Deputy Inspector James Grant.

But the copies obtained by The Post this week include exchanges that were redacted in court papers, with some emails showing how de Blasio praised Rechnitz for advising him to attend an anti-crime CompStat meeting at NYPD headquarters and later thanking him for the suggestion.

Another newly obtained email shows de Blasio thanking Rechnitz for tipping him off that the private Jewish Shomrim security patrol knew about the 2014 kidnapping and slaying of Menachem “Max” Stark two hours before the NYPD did.

“That’s a real issue. Thanks for raising it,” de Blasio wrote on Jan. 4, 2014.

Records released by City Hall also show that de Blasio forwarded Rechnitz’s email to then-Police Commissioner Bill Bratton and other officials.

Other laudatory emails from de Blasio came in response to hefty donations from Rechnitz.

On Feb. 4, 2014, de Blasio sent Rechnitz an email that said, “But want to profoundly thank you for all the help you’ve given lately. Means a lot to me.”

That show of appreciation came six days after Rechnitz gave $50,000 through his JSTD Madison LLC to the mayor’s now-defunct charity, the Campaign for One New York.

De Blasio also sent Rechnitz an Oct. 21, 2014, email consisting solely of a subject line that reads, “You’re a mensch!”

A day later, the state Board of Elections recorded a $102,300 donation from JSTD Madison to the state Democratic Senate Campaign Committee, for which de Blasio was soliciting contributions as part of a failed bid to flip the Republican-controlled state Senate.

The mayor also sent Rechnitz a May 28, 2014, email that consisted entirely of the subject line, “Will call you tmrw, my friend.”

When the call never materialized, Rechnitz wrote back late the next day, saying, “I guess we will touch base tomorrow?

“We’ll get there, brother,” de Blasio replied the following morning. “A lot of other issues have come up suddenly.”

Rechnitz returned to his native Los Angeles after secretly pleading guilty in June 2016 to wire-fraud conspiracy in a deal to assist the feds with several corruption probes.

Those investigations involved both de Blasio and his former campaign finance director, Ross Offinger, neither of whom was ever charged.

In October 2017 — after Rechnitz testified of de Blasio, “He took my calls. We were friends” — the mayor told reporters that they were only in contact “for a year, or a year and change.

“But this is not someone I ever knew well or was close to,” de Blasio said. “We were not close. He is exaggerating in many, many ways.”

He also called Rechnitz “just a horrible human being.”

During nine days of testimony that wrapped up earlier this month, Rechnitz claimed that he, Reichberg and former cabby advocate Fernando Mateo schemed to try to bribe public officials, including de Blasio and then-Westchester County Executive Rob Astorino.

Neither official was ever charged.

Still, the three men donated and raised more than $250,000 for de Blasio: $100,000 in campaign contributions, $50,000 for the Campaign for One New York and $102,000 for the Democratic Senate Campaign Committee.

Rechnitz also testified that he and Reichberg showered Grant and other cops with payoffs that included trips to the Dominican Republic, Las Vegas and Miami, with “hookers everywhere.”

Jurors even saw a video of Rechnitz and Reichberg wearing Santa hats as they rode around in a black Aston Martin convertible to deliver pricey gifts — including Nintendo games and American Girl dolls — to their cop buddies on Staten Island.

Rechnitz has testified that he doctored some emails between him and de Blasio, but the source who provided the emails to The Post vouched for their authenticity.

De Blasio has refused to say whether he deleted any of his ­emails with Rechnitz, and City Hall has offered a series of explanations for why other new ­emails surfaced in court, including a Bloomberg-era policy that they do not need to be archived.

Mayoral press secretary Eric Phillips has also argued that the de Blasio emails that surfaced in court “weren’t discovered in the search during the FOIL process.

“We have much more aggressive search protocols now than we did several years ago,” he told Politico on Dec. 6.

Last week, City Councilman Ritchie Torres (D-Bronx) called on the Department of Investigation to open a probe “into City Hall’s failure to fully disclose communications with Jona Rechnitz.”

Phillips on Monday declined to comment on the emails obtained by The Post.