You might have noticed that while adding your guests, some of them are highlighted in pale yellow color. This is simply to inform you that the email address for that guest is either empty or is an invalid email.
The reason this feature was added is to let you know that these guests may not receive an email invitation as their emails are invalid. By invalid, we mean the email is either empty or malformed (example: test@test or whatever@@test.com.random). We cannot guarantee that the email actually exists or not. You will however receive a summary email after sending out an invite which should tell you if there are any emails that were bounced.
