In general yes it is safe to make deposits to your bank account by mail, All of the major banks in the United States including Bank of America, Chase, Citibank, PNC, SunTrust, Wachovia, Washington Mutual and Wells Fargo accept deposits by mail and they don't usually charge a fee for this service while some smaller banks may charge a minimal fee, You should consult your bank's local branch for the exact terms of condition for their Deposit by Mail program.
When making a deposit to your bank account by mail you should follow these rules to assure that your deposit is received and processed in a timely manner.
1) DO NOT SEND CASH!!! In general banks do not accept cash deposits by mail, they will accept personal, business, official and certified checks and Money Orders such as those from another bank, the United States Postal Service or from a recognized financial services agency like MoneyGram International, Western Union or Amscot.
2) DO NOT ACCEPT CHECKS OR MONEY ORDERS FROM SOMEONE YOU DON'T KNOW. One of the biggest scams that is happening around the world is bogus checks and money orders are being mailed to unsuspecting dupes who are told that they one a lottery or other prize and that a check or money order is enclosed to cover the processing fees which need to be returned. These checks are 100% of the time are counterfeit and that YOU will be held responsible for the loss and a good portion of the time the bank will contact the police and have you arrested for attempting to defraud the bank. So only deposit a check or money order from someone who you are directly familiar with.
3) Mark your checks with the appropriate signatures and notices. When sending a check or money order by mail make sure you do three things, On the back of the check in BLACK INK ONLY on the first line put your Signature, On the second line put your Checking or Savings account number and on the third line put the words "FOR DEPOSIT ONLY" (Without the Quotes) which will protect you from having someone from stealing your checks and depositing them into their accounts.
4) Sending Your Deposits. Before you mail off the checks to your bank make sure that you have the correct mailing address, Most banks will either provide you with a pre-paid/pre-printed envelope or you can find the address on the bank's website either in the Contact Us section or in a section that is only accessible after logging into your online account.