Mail – especially financial documents – is a popular target for crooks.

Since May, the U.S. Postal Service Inspection Service has made more than 530 arrests for mail theft even as it steps up security measures to prevent thieves from making away with letters and packages.

Checks in particular are being targeted with thieves “washing” them to change the payee names and dollar amounts before fraudulently depositing them. Sometimes, thieves go as far as to steal checks from mailboxes and then wash them in chemicals to remove the ink though some scammers also use copiers and scanners to print fake copies of the documents.

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Experts recommend using a gel pen to help thwart thieves as the way ink is absorbed into a greater deal into the paper. Post Inspector Michael Martin said individuals can also prevent mail theft by not leaving uncollected letters in their mailbox for long periods of time or mailing checks at a post office as opposed to leaving in a box.

Best days to send checks, financial documents

There are certain days of the week that are better for mailing financial or other important documents., experts advise. In general, this means mailing financial documents earlier in the week, particularly Monday through Thursday to ensure quicker delivery times.

“In my experience, the best days to mail financial documents are early in the week — Monday, Tuesday or Wednesday,” Rahul Paragi, founder of business development site NamesPilot.com told GoBankingRates.com. “The postal service has fewer items to process on those days, so letters and packages tend to move through the system faster.”

Mailing on a Friday or Saturday or dropping off on Sunday could mean slower processing or more time sitting in the box – all of which can make documents more vulnerable.

People are also urged to track the status of incoming mail using Informed Delivery which sends a daily email with info on mail and packages they are to receive that day.