well i've just spent 3 hours wiki'ing, forum'ing and googl'ing and eventually found the answer to this - it's pretty cryptic (no pun intended)
request the cert in tomcat format, download the signed cert in x.509 format (you have to cut and paste into a file, call it commercial.crt).
download the root ca cert from here:
https://search.thawte.com/support/ss...INK&id=AR1470#
again, cut and paste into a file, call it ca.root
download the ssl intermediate bundle cert from here:
https://search.thawte.com/support/ss...LINK&id=AR1372
you'll want to choose the 'Apache, Plesk & CPanel' option 1, ie 'Download the Bundled CA version' - save it to ca.inter
now concatenate the two:
cat ca.root ca.inter >commercial_ca.crt
edit the newly created commercial_ca.crt and make sure that the ---BEGIN and ---END lines are all on their own line, and there are no empty gap lines anywhere.
then verify:
/opt/zimbra/zmcertmgr verifycrt comm /opt/zimbra/ssl/zimbra/commercial/commercial.key commercial.crt commercial_ca.crt
hopefully this goes ok - if you get any wierd errors you either have requested the cert wrong or have not quite got the above steps 100% correct.
then install:
/opt/zimbra/zmcertmgr deploycrt comm commercial.crt commercial_ca.crt
then restart zimbra