Sending email without actually signing in to gmail account from flutter app

I want to send user an email on order confirmation. For this I'm using mailer 3.2.1 package. https://pub.dev/packages/mailer

At below line in below code snippet, I'm getting prompt that saying 'gmail' is deprecated and shouldn't be used. Favor gmailUserXoauth2 as username/password is considered a Less-Secure-Apps.

       final smtpServer = gmail(username, password);

After replacing gmail with gmailUserXoauth2 in above line, I get The method 'gmailUserXoauth2' isn't defined

Below is the code snippet

String username = 'xxx@gmail.com';
String password = 'xxx';

       final smtpServer = gmail(username, password);
                      // Create our message.
                      final message = Message()
                        ..from = Address(username, 'Your name')
                        ..recipients.add('destination@example.com')
                        ..ccRecipients.addAll(
                            ['destCc1@example.com', 'destCc2@example.com'])
                        ..bccRecipients.add(Address('bccAddress@example.com'))
                        ..subject =
                            'Test Dart Mailer library ::  :: ${DateTime.now()}'
                        ..text =
                            'This is the plain text.\nThis is line 2 of the text part.'
                        ..html =
                            "<h1>Test</h1>\n<p>Hey! Here's some HTML content</p>";

                      try {
                        final sendReport = await send(message, smtpServer);
                        print('Message sent: ' + sendReport.toString());
                      } on MailerException catch (e) {
                        print('Message not sent.');
                        for (var p in e.problems) {
                          print('Problem: ${p.code}: ${p.msg}');
                        }
                      }

Upon googling this issue I have come across this solution. https://stackoverflow.com/a/58732280/7290043 This solution gives suggest following

1. Enable "Access for less secure apps" to use Gmail without OAuth 2.0
2. Sign in with Google account using the google_sign_in package and then send email

But I don't want to do either of these. I dont want to send email after signing in to google with google_sign_in package

What do I do now?