Try to digg inside the folder
/opt/zimbra/tomcat/webapps/zimbra/
js/
There you find the big Javascript Files that make up the User Interface
Application of the advanced webclient.
Also look into the subfolders.
And with a modification like described here:
12921-mini-crm-solution-show-notes-field-email-tooltip-popup
you COULD find a way to do it.
Or you write a Zimlet that reacts on all email messages, independent
from the emailaddress and the Logo could be displayed if the User
does a mouse over the email address. Would be less anoying
to the User, but less effective for the marketing effect.
You could also put some logic into the Zimlet, and display
HTML contenct depenging on the email address domain,
or depending on the logged in Zimbra Users email address
or domain name.