Yukatan data model 1.0

Yukatan data model project is the schema definition of the Yukatan webmail database.
The PostgreSQL database structures defined in this file can be used as a backend store of an email message handling application. The database should be created with the "UNICODE" encoding to properly support messages in different languages.
New data types
The special data types commonly used in the Yukatan data model have been made explicit by the introduction of seven new domains. The domains and the related COMMENT statements make field semantics more clear than before.
See the SQL schema file for more detailed documentation on these domains.
Explicitly named constraints
All the table constraints in the database are now explicitly named and documented. This change makes the database implementation more orthogonal and cleans up the documentation.
Renamed fields and tables
All the *address field names have been truncated to *addr, to make it visually clearer that they are always paired with the corresponding *name fields. The change also makes parts of the documentation less repetitive.
The referencesfield table has been renamed to referencefield to avoid the plural form in the table name. Also all the contained references* field names have been renamed to reference*.
Semantic changes
Quite a few changes have been made to the semantics of various fields. The unnecessarily tight constraints on sequence numbers have been replaced with clearer documentation, the format and encoding of most fields has been explicitly documented, and the previously allowed dual use of the enttext and enddata fields has been prohibited.
Dropped envelope data
The envelope data added in version 0.5 of the data model has for now been removed. The reason for the removal is that the envelope data is not an integral part of an email message, and I wanted to make the version 1.0 as clear as possible. The database now stores "email messages" - nothing less, nothing more. Envelope data can and probably will be reintroduced in an incremental version 1.x along with other extensions.
Enhancements:
- cleans up and documents the data model that has developed since version 0.1
- removal of the envelope data added in version 0.5
- enaming and redefinition of some of the fields and tables
- database structure has also been extensively documented