Thanks for your reply. It turns out the problem was an incorrect server name supplied by the service provider, which worked for web access but not mobile sync.
For the record, the Motorola Defy MB525 does not use the same settings as decribed in the Zimbra wiki for Android Smartphones (
Mobile Device Setup - Zimbra :: Wiki).
The setup is done via the "Corporate Sync" icon instead of via the normal "Mail" icon. The latter now only allows the creation of IMAP and POP accounts.
In the corporate sync settings:
The "Domain\Username" field is replaced by a simple "Username" field, so there is no need to experiment with including a backslash or not.
The "Exchange Server" field is now just called "Server".
The "Accept all SSL certificates" is now called "Verify Certificate", which should obviously have the opposite value to the old field.
It would be good if someone added these instructions to the wiki. Unfortunately I don't have the phone myself so I would not be able to do it accurately.