Nirsoft has another great utility you can download, called PSTPassword. PST archives employ very weak encryption. PSTPassword can identify three passwords which can open a .pst archive. Microsoft did not use a strong MD5 or SHA1 hash value, but instead employed a CRC32 algorithm which can be cracked with more than one password.
Nirsoft has posted a list of 17 passwords which will be interpreted as a CRC value in an Outlook .pst archive:
If the password for your .pst file is one of these, the second time you try to enter the password, Outlook won't prompt you to enter a password.