Gmail – have Gmail automatically add a prefix to the subject line when emailing a particular group

Here's a bit of generalization for a real-world scenario that's likely to occur...

I am a member in several groups that partially overlap. For some of those, setting up a Google group as suggested above is not applicable.

Some mails do not involve all the members, so simply including everyone in the to: section will not work either.

To differentiate and tag them I use filters with a disjunction of all group members in the from: section, and disjunction of partial-groups' conjunctions in the to: section, e.g:

If a group includes members {a,b,c,me}, I'll use:
  from:(a OR b OR c OR me) AND to:((a AND b) OR (a AND c) OR (a AND me) OR (b AND c) OR (b AND me) OR (c AND me))
Or shorthand:
  from:(a|b|c|me) to:((a b)|(a c)|(a me)|(b c)|(b me)|(c me))

Notes:

1. The space char, of course, defaults to AND, but for some reason the vertical bar (|) also works as OR, even though I found no documentation for it on Gmail.

2. For large groups I don't include all subgroups ("n choose k" can become quite big here :) ...in such cases I suggest using the most likely "sub-sub-groups":

   • those that have a specific function
   • or are more likely to interact with each other
   • or key members within the group that appear in most/all mails

   ...and then update your filter if you notice any mail that was not detected.

3. In the specific case where all group members are included in every email, the only sub-group is the entire group... using the above example's definitions that would be:
   from:(a|b|c|me) to:(a b c me).

4. Dropping the from: section altogether can be beneficial in cases where the group is contacted by "outsiders", i.e.: consultants/contributors/sources/dependents/etc.. in a specific project, for example:
   The (group of) event organizers is likely to stay largely constant from one project to the next, while the sub-contractors employed in any specific event may change.

   A word of caution! dropping the from: section can also lead to (more) false positives.

I would let one account handle the spam filtering for all of your accounts. I don't believe there are many benefits to having each account handle its own spam. I think Gmail is aware of the destination address of each incoming mail and having identical spam emails (with only the destination address changing) will strengthen the spam algorithm just as much as if it were single emails sent to multiple accounts.

Regardless of how much (more) it might improve the spam algorithm, you should choose the solution which best improves your workflow. I would just go with the first option (spam filter on the main account).

FYI: I use Gmail to process email from 12 accounts across 7 domains each with spam being primarily handled by my main Gmail account. :)