You can only email students for which you have a valid email address. There could be several reasons why a student cannot be emailed:
Student has no email address
You can add students to your list with just a first name. You might edit the student record and add more information later on (asides from email address, you can take notes on students, and even add a picture). But obviously, if there's no email address, the student cannot be emailed.
Student has an invalid email address
Ubindi detects if an email sent to a student bounces (meaning: fails to be delivered, usually because the address is misspelled or invalid, or sometimes because the student's mailbox is full or rejects emails for some other reason). When this happens, the email status for the student changes to "bad email" and they will be excluded from future email batches that you try to send.
The bad email status is cleared when you edit the student and change their email address in some way.
Student has unsubscribed
Every email you send out gets a small footer added, where the student has the option to unsubscribe from your emails. This is required by law, and it makes sure that Ubindi is not abused by teachers who send out unsolicited emails ("spam"). If for some reason one of your students opts out of receiving future email from you, they will then be excluded from any future emails you send out.
To see whether a student is subscribed you can click on a student's name on your list and go to their detail page. If the student has unsubscribed you'll see it in red at the top, near their name. Like so:
It's your responsibility to only add students to your list with whom you have an implicit understanding that it's OK to email them!
"Live" students vs. "Skeleton" students
When you add a student to your list yourself, they are considered "skeleton" students in the database. This means that they have not actively signed up to be your student, and they've not set a password for their Ubindi account.
You can edit "skeleton" student information as long as they are not "live": you can change their name, email address, and you can even add a picture of the student. For example, when you see a new face in class, you can add them to your student list, and maybe even take a photo of them using your phone (with their permission, of course!).
A "live" student is someone who has actively signed up as your student. They might do so in the process of registering for one of your classes, or they might just go to your public profile and "follow" you. Once a student is "live", they are in charge of editing their own information. Only they can change their email address or name, and only they can add or edit their picture.