No I use a public danish certificate ->
http://www1.certifikat.dk/producer/vis_side.pl?id=131&tmpl=23 called 'TDC OCES CA' the root-certificate can by found here ->
http://www1.certifikat.dk/repository/ocesca.crt
And there are no problems with the R5 client it accepts the certificate and I get and choice to make a cross-certificate with my NotesID into the PersonalNab the first time I get a mail signed by a user from this certificate.