The current generated code (only the POJO part and removed adapter for brevity):

EmailDummy.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public class EmailDummy {

  @Nonnull
  public final String From;

  @Nullable
  public final List<String> To;

  @Nullable
  public final List<String> CC;

  @Nullable
  public final List<String> BCC;

  @Nullable
  public final String Subject;

  @Nullable
  public final String Body;

  private EmailDummy(Builder builder) {
    this.From = builder.From;
    this.To = builder.To == null ? null : Collections.unmodifiableList(builder.To);
    this.CC = builder.CC == null ? null : Collections.unmodifiableList(builder.CC);
    this.BCC = builder.BCC == null ? null : Collections.unmodifiableList(builder.BCC);
    this.Subject = builder.Subject;
    this.Body = builder.Body;
  }

  @Override
  @SuppressWarnings("StringEquality")
  public boolean equals(Object other) {
    if (this == other) return true;
    if (other == null) return false;
    if (!(other instanceof EmailDummy)) return false;
    EmailDummy that = (EmailDummy) other;
    return (this.From == that.From || this.From.equals(that.From))
        && (this.To == that.To || (this.To != null && this.To.equals(that.To)))
        && (this.CC == that.CC || (this.CC != null && this.CC.equals(that.CC)))
        && (this.BCC == that.BCC || (this.BCC != null && this.BCC.equals(that.BCC)))
        && (this.Subject == that.Subject || (this.Subject != null && this.Subject.equals(that.Subject)))
        && (this.Body == that.Body || (this.Body != null && this.Body.equals(that.Body)));
  }

  @Override
  public int hashCode() {
    int code = 16777619;
    code ^= this.From.hashCode();
    code *= 0x811c9dc5;
    code ^= (this.To == null) ? 0 : this.To.hashCode();
    code *= 0x811c9dc5;
    code ^= (this.CC == null) ? 0 : this.CC.hashCode();
    code *= 0x811c9dc5;
    code ^= (this.BCC == null) ? 0 : this.BCC.hashCode();
    code *= 0x811c9dc5;
    code ^= (this.Subject == null) ? 0 : this.Subject.hashCode();
    code *= 0x811c9dc5;
    code ^= (this.Body == null) ? 0 : this.Body.hashCode();
    code *= 0x811c9dc5;
    return code;
  }

  @Override
  public String toString() {
    return "Email{From=" + this.From + ", To=" + this.To + ", CC=" + this.CC + ", BCC=" + this.BCC + ", Subject=" + this.Subject + ", Body=" + this.Body + ", Attachments=" + "}";
  }

  public static final class Builder {
    private String From;

    private List<String> To;

    private List<String> CC;

    private List<String> BCC;

    private String Subject;

    private String Body;


    public Builder() {
    }

    public Builder(EmailDummy struct) {
      this.From = struct.From;
      this.To = struct.To;
      this.CC = struct.CC;
      this.BCC = struct.BCC;
      this.Subject = struct.Subject;
      this.Body = struct.Body;
    }

    public Builder From(String From) {
      if (From == null) {
        throw new NullPointerException("Required field 'From' cannot be null");
      }
      this.From = From;
      return this;
    }

    public Builder To(List<String> To) {
      this.To = To;
      return this;
    }

    public Builder CC(List<String> CC) {
      this.CC = CC;
      return this;
    }

    public Builder BCC(List<String> BCC) {
      this.BCC = BCC;
      return this;
    }

    public Builder Subject(String Subject) {
      this.Subject = Subject;
      return this;
    }

    public Builder Body(String Body) {
      this.Body = Body;
      return this;
    }

    public Builder Attachments(List<Attachment> Attachments) {
      if (Attachments == null) {
        throw new NullPointerException("Required field 'Attachments' cannot be null");
      }
      return this;
    }

    public EmailDummy build() {
      if (this.From == null) {
        throw new IllegalStateException("Required field 'From' is missing");
      }
      return new EmailDummy(this);
    }

    public void reset() {
      this.From = null;
      this.To = null;
      this.CC = null;
      this.BCC = null;
      this.Subject = null;
      this.Body = null;
    }
  }
}