This is a bit of a shot in the dark as I can’t regularly replicate this issue, but I’ve had multiple people indicate they’ve had problems.
When a form is submitted with an email address entered, it will occasionally return an error that says “Invalid email address: firstname.lastname@example.org” even though the entered field is clearly an email address. If the user immediately hits submit, it will send just fine without having to re-type the email address.
I am having trouble figuring out exactly what could be causing it. So far I have determined that it’s not a caching plugin, not a captcha issue (happens on forms without captcha), and it doesn’t happen every time. I am just not finding a common thread here, but maybe someone else has encountered something similar and can provide insight?