Calendar entries that live in Exchange 2010 cannot be edited
I was able to retrieve the primary e-mail address from my phone: In the details of an appointment, I was listed as attendee under an e-mail address different from the one the account was configured for, and the e-mail address of this attendee turned out to be my primary e-mail address.

I was unable to change the e-mail address for the existing account. After dropping and recreating the account using the primary e-mail address, I can now update details of all appointments on my device.