https://web.archive.org/web/20061209042018/http://trac.mulberrymail.com/mulberry/wiki/olderversions

Registration for older versions of Mulberry ¶
Older versions of Mulberry require a serial number and registration code to use. Below are codes you can use with these older versions now that Mulberry is free.

Mulberry Version	Serial Number	Registration Code	Security Plugin Key
v4.x			SFREE		D71B 79CC C592 6E60	Not required
v3.x			SFREE		8B37 34EE 2324 08CE	B69B509A

Any '0's in the registration codes are the numeral zero.
Security plugins (PGP etc) prior to v4 required an extra code (provided above).