By receiving e-mails it would be necessary to have to option to select multiple e-mails and link them to one deal. How is it possible?
If you add the person to the deal as a participant it will link emails to that deal. You can then see all communications with anyone involved in on the deal page.
Unfortunately, I'm not aware of a way to link/unlink multiple emails manually via the Web UI. If auto-linking doesn't do the trick (e.g., if the contact has multiple deals) it's just lots of clicking...
Imho it would be great already, if we could change the deal link directly in the list in the inbox, instead of having to descend to the details of every single email!