How do validate an entered e-mail adress (JSF dialog)?

You can use bean validation of type regex to validat mail addresses. I've created a demo IAR here.

The approach is quite simple. I use a normal java bean with an annotated e-mail field. This java class is used on a JSF dialog and therefore annotations are automatically validated.

public class Person 
{
  /**
  * @see http://www.regular-expressions.info/email.html
  */
  @Pattern(message="mail address is invalid", 
        regexp="[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?")
  @NotNull(message="<%=ivy.cms.co(\"/ch.ivyteam.htmldialog.demo/BeanValidationDemo/notnull\")%>")
  private String email;

  public String getEmail() {
    return email;
  }
  public void setEmail(String email) {
    this.email = email;
  }

Bean validation compared to in view validation (as shown by roland) has an advantage. You can re-use the validation on every dialog where you use the bean without writing extra code. So define validation once, have a valid state everywhere...