I know, we may be used to think different with Helpdesk Software than OpenERP does :) But it works, I try to explain, how i got happy with this. Its still no replacement for other Helpdesk Software. But you can work with it:
Open
If you have proper installed and configured the helpdesk module and the automatic creation of an new issue due an incoming e-mail: Open your new automaticly created issue called "YourIssue" (Project -> Issues -> "YourIssue"). You have to use the Menu "Projects". Not via Partner/Contact.
Reply
In the open Issue, on the Bottom, above the History, you can click to "Send a message". This is how you answer this E-Mail / Issue. Yup, you'll be missing the E-Mail Template and the "We received your message, your Ticket ID is: xyz". Take a look at this: https://accounts.openerp.com/forum/Help-1/question/31783/
If you just have to add an internal note, use "Log a note". Jup, i also expected some big "reply-buttons", but this is how the communication system in OE works.
Logged History
Below the "Send a message" Button, there is a complete History of this Issue...I know, its not filtered to E-Mails only :( But if you click to "follow" -> "Discussion", you will receive further mails. Yes, it is still not a filtered history of the past...
This may be an interessting explantaion: https://accounts.openerp.com/forum/Help-1/question/35894/. There may be an approach
No, as far as i see, this Issue and all its History is "not yet" linked to the customer/partner.
If this helped a little bit and answered your question, please vote. It is just a little summary of ideas and answer to your question. it's not helpdesk replacement guide ;)
Would have thought this is functionality many people use....??
You can send any massage. Just put email in Recipients.
If you've every used any kind of help desk software at all (fogbugz, zendesk, freshdesk, etc.) you'll understand my point. If I'm looking at a partner, how do I see all email communications with them, that have been through the help desk?