I have been using the following (partly anonymized) to send email from
one of my accounts:

(defun srb-rub-smtp-send-mail-setup ()
  ""
  (interactive)
  (makunbound 'message-send-mail-function)
  (makunbound 'smtpmail-default-smtp-server)
  (makunbound 'smtpmail-auth-credentials)
  (setq message-send-mail-function 'smtpmail-send-it
	smtpmail-default-smtp-server "mail.rub.de"
	smtpmail-smtp-service 587
	user-mail-address "xyz@rub.de"
	smtpmail-auth-credentials
	'(("mail.rub.de" 587 "username" "password"))
	smtpmail-starttls-credentials
	'(("mail.rub.de" 587 nil nil))))

Note that no certificate and key files are specified for
smtpmail-starttls-credentials.  This code has worked fine until
recently.  Now when I use it no mail is sent and I get the following in
*Messages*: