The reason for this is usually because of 1 of 2 things that we are working to address:
Your email address has a number in it. At the moment there is no way round this and if you can find and use an alternative address till we get this sorted, that would be great.
There is a "whitespace" in the email field. A whitespace is what is created when you hit the spacebar on your keyboard. So just make sure there are no whitespaces before or after your email address, and if there are, just delete them.
Example: " firstname.lastname@example.org" or "email@example.com " - if you can spot the coloured gap between the " and the start and end of each mail address in this example then you'll have spotted how to sort this issue out.