As Dkay suggested you can do the email address validation at the UI level. However, if there is something specific (specially on receiving email) then you can do that in plugin as well.
1. You will need to register the plugins in the Pre-event as you are trying to perform some validation.
2. For the incoming emails, there are certain messages like CheckIncoming, CheckPromote, DeliverIncoming, DeliverPromote & Create that you may need to register for.
3. For outgoing emails you can use the Send message.
You can get the complete list of plugin messages from this
post. These messages are not amongst the frequently used and I'm not sure of any limitations on these. Pls test the plugins on all possible scenarios.
Hope this helps!
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