Personally I feel the insertion of clean notes is of little value, who would trust them, certainly not me.
If I receive an email with such a clean note from a person using another AV, would I trust it, not a chance, so why would I expect another user to trust my email just because it has a clean note attached, I wouldn't, so there is little vale there.
If I receive an email witch happens to be infected, all hell breaks loose with avast displaying a visual and audible alert. So what use are clean notes, if an email isn't infected avast doesn't alert, then it is just as clean as if there were a clean note inserted, ut again I take nothing at face value and always exercise safe hex.
So even with avast 5.0 which should be able to handle SSL traffic, in the same way as STunnel, I don't know if it will be able to insert a clean note (quite possibly), but it still doesn't detract from the question, 'what is the worth of inserting a clean note,' very little.
Really the only benefit is in sending email it effectively promotes avast, but it can't be taken as gospel that it is clean. There are many malware variants that write their own fake clean note into the spam they send, it lulls people into a false sense of security.
No you shouldn't turn it off, in fact you should set the sensitivity to High, as trojan spambots come with their own SMTP function and avast would be the only thing to scan it and it should alert you to having multiple identical email in a period of time. This may be your first indication that you have an undetected or hidden spambot (often come with a masking program) being on your syste.
