As that website I linked says, if the email looks suspicious but is technically valid, ask them "Are you sure you didn't mean this?", but let them choose what they put in.
However, if the address as given is definitely invalid, then you can reject it and explain why. You can offer suggestions here too if any seem reasonable.
Comments
For UX purposes you would use a warning/extra confirmation. If you make the decision to block the user from registering, you are doing something else.
Yeah, precisely.
As that website I linked says, if the email looks suspicious but is technically valid, ask them "Are you sure you didn't mean this?", but let them choose what they put in.
However, if the address as given is definitely invalid, then you can reject it and explain why. You can offer suggestions here too if any seem reasonable.