It is possible to write a check for $1m and have it accepted though such transactions are rare because of the amount of money involved. Your bank would want to ascertain the source if the money and if the check is authentic.
Federal law stipulates that a bank can hold a check deposit for seven business days for verification. However, upon deposit of the $1 million check, the bank is expected to provide a total of $5000 within two business days pending verification. This is broken down as $100 within the next business day after the check has been deposited, and $4,900 after two business days.