Hi!

Make sure that you include the whole email address before you pay attention to the message.

If you have already done that, ensure that the email is not already in use. If you have already created an account, follow the instructions described here to set it up again: https://support.mozilla.com/en-US/kb/add-a-device-to-firefox-sync#w_how-do-i-add-a-device-when-im-not-near-my-computer

It tells me my email address is not valid after 1 letter

Are you trying to open an acoount (i.e. starting from the beginning) or you already have an account and want to activate sync on another device?

Trying to open a new account

Do you still see that message if you type or paste the full email address?
You see that error message if there isn't a valid domain (@server.xxx) present.

Did you ever use that email address before to open a sync account?