- Doubleclick on the TextBox or left click and choose Options
- Click DataValidation
- Click Add
- In the first dropdown select the TextBox (selected as default)
- In the second dropdown select "Does not match pattern"
- In the third dropdown select Userdefined pattern and enter the pattern.
Here is a growing list of patterns.
It's not perfect, because it allows a.b.@c.com and email@example.com., but it does allow firstname.lastname@example.org and email@example.com