Not checked this one out much, but might be worth a shot.

http://www.npemap.org.uk/data/

We've used this lot who provide it for free, or with a premium service but don't think they can do anymore than the first part of the postcode, eg: CA11

http://www.geonames.org/

The best database is the one you licence from the Royal Mail or one of it's licensors, but best also equals expensive....very expensive.

Sendmail work with mysql

but you need other DB you must change config file

It depends on how much you want to spend and what level of detail you require. Datasets are OS independant so it does not matter which operating system you use. You may need to convert it to mySQL or which ever database server you are using though.

If you want free then the afore mentioned datasets at http://www.npemap.org.uk/data/ are probably your only options. I would use the dataset which includes freethepostcode.

If you are so inclined, part of the PAF dataset was leaked to wikileaks a few years ago. Unfortunately I do not know any more than that.

The database at http://linuxbox.co.uk/postcode_database.php sounds interesting but, unlike freethepostcode, they make no statement on where the data came from and how they can ensure that it does not infringe on Royal Mail's copyright.

Seen this one before ?

http://linuxbox.co.uk/postcode_database.php