You can use the following or modified version of the code below:
select email_id, character_length(email_id) as email_length,
case
when email_length >15 then 'SUSPICIOUS'
else 'NON-SUSPICIOUS'
end as email_type
from
<TableName>
where
email_type = 'NON-SUSPICIOUS'
Let me know if it worked for you.
Cheers,
Pramod