In all honesty this is a greatly depreciated option (see below), so I would imaging that it isn't in any of the avast versions and as far as I'm aware the Mail Shield functionality is the same in all versions.
Greatly depreciated in that it is easy to fake and these fake 'email scanned by XXXX (insert AV of choice) and found to be clean have been used in malicious emails in the past and who trust tat anyway, I certainly don't.
Why is it needed when if an inbound email is infected avast will alert, brand the email Subject and remove any infected attachment, etc. For outbound email it too would alert if infected/suspicious, etc. the only real benefit is promotion of avast.
For me the thought of going backwards to an older version of avast with a lessor degree of protection, for something which provides zero extra protection for what are cosmetic reasons, is crazy.