Very interesting, I have never seen that.
But isn't that the Outbound message having been branded and you seeing it at the bottom of the email, if you also had the Inbound message option checked, wouldn't you have two messages ?
I have never seen the point of branding inbound emails if they were infected I would expect the audible/visual alert popup. For a very long time I used the outbound message as a means of spreading the word to others about avast
Do you have any changes in the C:\ProgramData\AVAST Software\Avast\EmailShield.ini, I have some in mine, but I don't know if they are historic settings having modified the .ini file some time ago.