The ONLY file type for your certificate that the handset will read is a DER encoded binary X.509 format and save it with .cer extension. No other type of format will be read by the handset.
I've just extracted, copied to an XT variant 6120 (FW 6.01) and installed on the handset no issues.
From here you could try a hard reset of the handset to try and remove any erroneous files that may be hampering what you're trying to do, make sure you back up any important information first (contacts, text messages etc) and try starting again.
Have you tried contacting the Nokia Careline for help also? Being your issue appears to be somewhat involved I would suggest the written form would be a better approach. http://www.nokia.co.nz/support/ask-nokia
@BrentR: I am aware of the constraints around format. I think I mentioned that I was able to install another certificate to the phone. I actually did it in two ways: first in the DER encoded format you mentioned, and secondly with a PKCS#12 package containing the server certificate and a personal certificate. I can confirm other formats such PKCS#7 and base64 encoded X.509 don't work...
Cheers everyone for your advice. At this point I am aware of a number of means of transfering certificates to the phone's certificate store. None have yet worked for this specific certificate, but I'm not giving up yet! Here is my plan of attack: 1. The certificate is actually due for renewal in about a month's time, so we (dad and I) will get in contact with the certificate creator and ask them to renew it for us. If the new certificate works then all is well and we put the problem down to a single bad certificate, otherwise... 2. I have had a private offer of help from someone who has a 6120. They have offered to attempt to put the certificate onto their phone and/or attempt to reach the server. I will take them up on their offer at that point. If they are able to get the certificate onto their phone and sync with the server... 3. Backup and firmware upgrade. Still not working... 4. Talk to Nokia. No help... 5. Talk to 3rd party IT service provider who set up the server (including IIS, Exchange, NetScreen, security, certificates). No help... 6. Talk to Telecom. No help... 7. GIVE UP!
Note that I'm leaving Telecom until last because I don't think the problem is anything to do with XT, and although the phone is on contract I don't know any local agents that won't simply look at the Telecom 6120 website/advertising (which indicates that syncing to Exchange doesn't work) and say "it won't work".
I just thought I will share with you all know that 6120C is my work phone and I was able to successfully install Mail For Exchange 3.0 without any issues. I have successfully synchronised my email and am able to send and receive emails. Also, I receive emails on the device without any delay.
Hi again everyone. I've been very busy over the last couple of days but I finally got the chance to spend some more time on the problem this evening. Another reason for the delay has been the wait for the original certificate creator to get back to me (about cert renewal). They still haven't responded to my calls/emails and I was interested to learn more about certificates so I took it upon myself to do the job. The instructions I found via Google were for creating (rather than renewing) a self-signed certificate; I decided this would be okay (maybe even preferred) since we seem to have a problem with the current cert and nobody really uses it anyway. After 15 minutes of reading and fiddling with OpenSSH I had a new pkcs#12 ready for the server with the corresponding cer for the phone. I transferred the cer file to the phone first via USB and attempted to open it in the file browser... and hey presto the cert could be opened and saved to the phone's certificate store! "This is going well", I thought. I then backed up the server's current certificate and installed the new one. Next step was to check whether the phone could reach the server... and it could!!! After a few appointment duplication issues I now have the phone sync'ing calendar and contacts perfectly via Mail For Exchange...
My sincere and grateful thanks to all who have shared their advice and time - I *really* appreciate it (my dad appreciates it even more if that is possible). Hopefully I can return the favour sometime!