XBASE is a www4mail command used to set the base href applicable to URLs
contained in the e-mail message.

Example:
XBASE http://www.ictp.trieste.it/
XGET ~www4mail

The above two lines would send the www4mail processed URL of
http://www.ictp.trieste.it/~www4mail

XBASE also affects GET, SEND, SOURCE, TSOURCE, XSOURCE

Note: An e-mail message may not have multiple XBASE statements. 
Multiple XBASE statements are handled by using the last and
ignoring the others.