The problem here is symbols won't show in alert boxes. The solution? you need to type its unicode value instead of the symbol itself.
So for @ symbol, its unicode value is \u0040.
For example, instead of...
alert('contact us at firstname.lastname@example.org');
The code must be like this...
alert('contact us at customer\u0040service.com');
Note that not only symbols have unicode values, other characters also have such as letters.
To convert characters to unicode, click here.