dbmail_headername & dbmail_headervalue q uestion

Hi.

I got a question about the caches:

I just truncuated my tables to get rid of - several tens of thousands - 
of rows in them.

Now I was wondering what happens if I forgot some header that i 
should've kept in there? - Will my client just not receive any results 
for e.g. a search?

Also is there a good way to find out which headers my clients use and 
make sure I have them in dbmail_headername?

Regards,
   Thomas R