SMTP Auth after update from 7.4 to 7.6

[linit000]

Hi,

our SMTP Server (sendmail) is configured with TLS and various AUTH MECHS for all kind of Email Clients.
OX Server is configured to use no authentification and no encryption using the same sendmail Server. In 7.4 this setup works.
After update to 7.6 it is not possible to send emails even com.openexchange.smtp.smtpAuthentication in smtp.properties ist set to false.
Sendmail is providing following AUTH MECHS:
(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
The only way to send mails is to provide no MECHS like:
(`confAUTH_MECHANISMS', `')
But that's not an option, because other clients have to authenticate.
The changelog show only changes in ciphers.

Errorlog:

com.openexchange.exception.OXException: MSG-1030 Categories=PERMISSION_DENIED Message='Wrong or missing login data to access mail transport server internalsmtp.local . Error message from mail transport server: 535 5.7.0 authentication failed
' exceptionID=-328582521-17
	at com.openexchange.exception.OXExceptionFactory.create(OXExceptionFactory.java:164)
	at com.openexchange.exception.OXExceptionFactory.create(OXExceptionFactory.java:134)
	at com.openexchange.mail.mime.MimeMailExceptionCode.create(MimeMailExceptionCode.java:683)
	at com.openexchange.smtp.SMTPTransport.sendMailMessage(SMTPTransport.java:833)
	at com.openexchange.mail.transport.MailTransport.sendMailMessage(MailTransport.java:157)
	at com.openexchange.mail.MailServletInterfaceImpl.sendMessage(MailServletInterfaceImpl.java:2740)
	at com.openexchange.mail.json.actions.NewAction.performWithUploads(NewAction.java:286)
	at com.openexchange.mail.json.actions.NewAction.perform(NewAction.java:138)
	at com.openexchange.mail.json.actions.AbstractMailAction.perform(AbstractMailAction.java:196)
	at com.openexchange.ajax.requesthandler.DefaultDispatcher.perform(DefaultDispatcher.java:214)
	at com.openexchange.ajax.requesthandler.DispatcherServlet.handle(DispatcherServlet.java:388)
	at com.openexchange.ajax.requesthandler.DispatcherServlet.doPost(DispatcherServlet.java:313)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
	at com.openexchange.ajax.AJAXServlet.service(AJAXServlet.java:526)
	at com.openexchange.ajax.SessionServlet.service(SessionServlet.java:364)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
	at com.openexchange.ajp13.coyote.AjpProcessor.process(AjpProcessor.java:919)
	at com.openexchange.ajp13.coyote.sockethandler.CoyoteTask.call(CoyoteTask.java:183)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at com.openexchange.threadpool.internal.CustomThreadPoolExecutor$Worker.runTask(CustomThreadPoolExecutor.java:817)
	at com.openexchange.threadpool.internal.CustomThreadPoolExecutor$Worker.run(CustomThreadPoolExecutor.java:844)
	at java.lang.Thread.run(Thread.java:701)
Caused by: javax.mail.AuthenticationFailedException: 535 5.7.0 authentication failed

	at com.sun.mail.smtp.SMTPTransport$Authenticator.authenticate(SMTPTransport.java:843)
	at com.sun.mail.smtp.SMTPTransport.authenticate(SMTPTransport.java:765)
	at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:687)
	at javax.mail.Service.connect(Service.java:345)

So I guess, in 7.6 the default javamail properties are different.
Any hints?

Chris