A customer has raised an interesting point about gmail addresses. See: https://support.google.com/mail/answer/7436150?hl=en
From this link:
For example, if your email is email@example.com, you own all dotted versions of your address:
We’d like to know if/how Campaign handles this. Is there something OOTB that helps distinguish accounts from dotted or non-dotted versions? Or is it up the customer to setup some sort of regex to capture the right account (if so, has anybody set something like this up before)?
ACS does not do special cases for email addresses internally. Dots are considered as normal characters and are never ignored, as this is clearly a violation of the RFC.
As dots don’t matter in Gmail addresses, hence the same rule is followed on campaign side as well.