From the excellent Mail For Exchange (MfE) Heartbeat and FAQ;

"Keep in mind - self-signed certificate shall have "CA" field. Otherwise, N900 certificate manager will not allow to install it."

I guess my certificate has no CA (or a CA from an untrusted authority) and so it won't install and thus doesn't appear in the Certificates Manager... nothing I can do about it.

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After I moaned my company bought an SSL certificate and it all works wonderfully and was so easy to set up.