With 64,000 migrants shuttling through a warren of shelters in New York City, one of the city’s more difficult challenges is among the more unheralded: delivering the mail.

The influx of migrants from the southern border has brought a barrage of mail to the city’s more than 200 migrant shelters, overflowing the makeshift mail rooms in repurposed hotels and office buildings where the new arrivals are staying.

Some of the correspondence is critical: immigration notices or documents to apply for Social Security numbers and work authorization, documents that, if lost or delayed, would hamper the migrants’ ability to work legally in the United States.

But problems have arisen. Mail sometimes goes missing. The Postal Service has, at times, determined that mail is undeliverable at some shelters, such as the tent dormitory on Randall’s Island, according to immigration lawyers.

And with the city adopting a stricter policy that forces migrants to reapply for shelter, sometimes as often as every 30 days, migrants said they have struggled to find their mail.

Ever since New York City set up a multibillion-dollar emergency response to shelter and feed thousands of migrants two summers ago, officials have insisted that the way out of the crisis is to make migrants self-sufficient.