No, it shouldn’t… I mean, avast should detect the email format and add the specific note, html or txt.
So, something in this recognition or in the emails (are you sure they’re just plain text?) that is not working well.
Well, this are my guesses :
When avast! inserts info message into email, MAPI function for synchronizing RTF version and Plain version of
text is called. I think that if your email contains both HTML/RTF and plain parts, Outlook will show you “rich” version instead of plain text version. May be that if HTML/RTF part is not present, synchronization will create it (in some cases), so please try “magic” option “UseAlternateRtfSyncing” in avast! ini file (more info http://forum.avast.com/index.php?topic=1647.msg10233)