Lañaña

public class MailConfiguration {

  private AddressesPart addressesPart;

  private String subject;

  private FilesAttachments filesAttachments;

  private String bodyPart;

  public MailConfiguration(AddressesPart addressesPart, String subject, FilesAttachments filesAttachements,
    String bodyPart) {
    Validate.notNull(addressesPart, "addressesPart must not be null");
    Validate.notNull(subject, "subject must not be null");
    Validate.notNull(filesAttachments, "filesAttachments must not be null");
    Validate.notNull(bodyPart, "bodyPart must not be null");
    this.addressesPart = addressesPart;
    this.subject = subject;
    this.filesAttachements = filesAttachements;
    this.bodyPart = bodyPart;
  }
  // ...  some useful getters ......

}

public class AddressesPart {

  private final String senderAddress;

  private final Set recipientToMailAddresses;

  private final Set recipientCCMailAdresses;

  public AddressesPart(String senderAddress, Set recipientToMailAddresses, Set recipientCCMailAdresses) {
    validate(senderAddress, recipientToMailAddresses, recipientCCMailAdresses);
    this.senderAddress = senderAddress;
    this.recipientToMailAddresses = recipientToMailAddresses;
    this.recipientCCMailAdresses = recipientCCMailAdresses;
  }

  private void validate(String senderAddress, Set recipientToMailAddresses, Set recipientCCMailAdresses) {
    AddressValidator addressValidator = new AddressValidator();
    addressValidator.validate(senderAddress);
    addressValidator.validate(recipientToMailAddresses);
    addressValidator.validate(recipientCCMailAdresses);
  }

  public String getSenderAddress() {
    return senderAddress;
  }

  public Set getRecipientToMailAddresses() {
    return recipientToMailAddresses;
  }

  public Set getRecipientCCMailAdresses() {
    return recipientCCMailAdresses;
  }

}

public class AddressValidator {

  private static final String EMAIL_PATTERN
    = "^[_A-Za-z0-9-]+(\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\.[A-Za-z0-9]+)*(\.[A-Za-z]{2,})$";

  public void validate(String address) {
    validate(Collections.singleton(address));
  }

  public void validate(Set addresses) {
    Validate.notNull(addresses, "List of mail addresses must not be null");
    for (Iterator it = addresses.iterator(); it.hasNext(); ) {
      String address = (String) it.next();
      Validate.isTrue(address != null && isAddressWellFormed(address), "Invalid Mail address " + address);
    }
  }

  private boolean isAddressWellFormed(String address) {
    Pattern emailPattern = Pattern.compile(EMAIL_PATTERN);
    Matcher matcher = emailPattern.matcher(address);
    return matcher.matches();
  }
}

public AddressValidator(IValidator myValidator) {
   this.validator = myValidator;
}

this.senderAddress = requiresNonNull(senderAddress);

this.recipientToMailAddresses = validate(new HashSet<String>(
        recipientToMailAddresses
    ));