Newsletter send is not synchronous, we can't be sure if all mails are send

[MrTango]

When sending emails on Plone 5.2 and Waitress, the newsletter says all mails send out, but in fact there could be errors in the send process.

2019-12-04 13:41:00,787 INFO    [Products.EasyNewsletter:152][waitress] Newsletter was sent to (272) receivers. (0) errors occurred!
2019-12-04 13:41:15,536 WARNING [waitress.queue:122][MainThread] Task queue depth is 4
2019-12-04 13:42:20,845 WARNING [waitress.queue:122][MainThread] Task queue depth is 5
2019-12-04 13:43:25,518 WARNING [waitress.queue:122][MainThread] Task queue depth is 6
2019-12-04 13:44:30,212 WARNING [waitress.queue:122][MainThread] Task queue depth is 7
2019-12-04 13:45:35,231 WARNING [waitress.queue:122][MainThread] Task queue depth is 8
2019-12-04 13:46:12,177 ERROR   [MailDataManager:90][waitress] Failed in tpc_finish for <bound method SMTPMailer.send of <zope.sendmail.mailer.SMTPMailer object at 0x7f77e8d716d0>>
Traceback (most recent call last):
  File "/home/pacs/ita00/users/www/buildout/eggs/zope.sendmail-5.0-py3.7.egg/zope/sendmail/delivery.py", line 84, in tpc_finish
    self.callable(*self.args)
  File "/home/pacs/ita00/users/www/buildout/eggs/zope.sendmail-5.0-py3.7.egg/zope/sendmail/mailer.py", line 99, in send
    connection.starttls()
  File "/home/pacs/ita00/users/www/python/lib/python3.7/smtplib.py", line 785, in starttls
    raise SMTPResponseException(resp, reply)
smtplib.SMTPResponseException: (421, b'4.4.2 h82.bla.net Error: timeout exceeded')
2019-12-04 13:46:12,200 ERROR   [MailDataManager:90][waitress] Failed in tpc_finish for <bound method SMTPMailer.send of <zope.sendmail.mailer.SMTPMailer object at 0x7f77e8c7be10>>
Traceback (most recent call last):
  File "/home/pacs/ita00/users/www/buildout/eggs/zope.sendmail-5.0-py3.7.egg/zope/sendmail/delivery.py", line 84, in tpc_finish
    self.callable(*self.args)
  File "/home/pacs/ita00/users/www/buildout/eggs/zope.sendmail-5.0-py3.7.egg/zope/sendmail/mailer.py", line 99, in send
    connection.starttls()
  File "/home/pacs/ita00/users/www/python/lib/python3.7/smtplib.py", line 785, in starttls
    raise SMTPResponseException(resp, reply)
smtplib.SMTPResponseException: (421, b'4.4.2 h82.bla.net Error: timeout exceeded')