In new instances, at least for awhile, there was an Reset Invalid Campaign that triggers when Email Address is changed and then flips Email Invalid=F.

Yes, you should do this if not done already. Ok to be a batch too.

Grégoire, forgive me if this is a rookie question, but what does the trigger Campaign Is Requested do in this scenario?

Hi Greg,

This discussion is great, thanks for the screenshots for the trigger. I am setting that up now in our instance. Do you have to have another batch or rigger campaign running in the background to coorespond with the Campaign Is Requested trigger? I am also new to using that trigger.

Thank you!

Hi Eric,

Better create a new question than reopening an old one to get a quicker answer.

One way to homogenize all email addresses in lowercase is to use a webhook.

-Greg

Should I de-dupe my list before importing to make sure I'm not importing any leads that already exist? 

That's not going to be a solution because it means you can't merge new data in (other than the email address).

 Or always ensure that my email addresses are in lower case?

If you can "canonicalize" your email addresses everywhere they come in, sure.  This means you have to enhance your forms in the same way:

MktoForms2.whenReady(function(form)
  form.onSubmit(function(form){
    form.setValues({ Email : form.getValues().Email.toLowerCase() });
  });
});

Like Greg says, you can use a webhook to canonicalize any of your Marketo fields, though it's actually more complex to do this with Email Address in particular than with any other field (it does work, though).

In this scenario, you'd trigger your Smart Campaign on Webhook Is Called, and if the webhook says, "I cosmetically fixed up the Email Address to make it lowercase, but the fixed-up version is functionally identical to the last value" you don't change the Email Invalid status, otherwise it's a real change and you flip Invalid.

Interesting factoid about email address case-sensitivity: the local part of an address (the part on the left of the "@", or the mailbox) is explicitly defined as case-sensitive in the SMTP standard (the domain part, on the right, is separately defined as case-insensitive under the DNS standard). RFC 5321 (the current standard) is unequivocal on the matter:

The  local-part of a mailbox MUST BE treated as case sensitive.  Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. In particular, for some hosts, the user  "smith" is different from the user "Smith".

Nevertheless, this doesn't stop people from lowercasing (or, in certain banking settings, uppercasing) email addresses. It's technically wrong, but the risk is low enough that few people ever see an issue. Unfortunately, it's impossible to be case-preserving and to merge/search case-insensitively.at the same time. Only one of these can be true, so 99.999% of people decide to treat addresses as case-insensitive. It's interesting (to me, at least, as a longtime mail admin) that they're actually not!

Finally (also like Greg said!) please open a new thread in Products to discuss this stuff further. It's better for later Community searches, and also enables you to mark an answer Correct.