after upgrading from from 1.4.13. to 1.6.5, I can not send an email

[linpi]

I use rouncube since many years on a small linux server. I tried to upgrade from 1.4.13. to 1.6.5 and the upgrade worked well. But know there is an error message when I'm trying to send a email:

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SMTP error (530): the sender 'test@example.com' could not be set (5.7.0 Must issue a STARTTLS command first).I had to go back to 1.4.13 since the server is used from about 50 people/friends and I can not block it too long.
In the rouncube version 1.4.13, the smtp server was empty: $config['smtp_server'] = ''; and was and still working fine...
Know in the version 1.6.5 there is a $config['imap_host'] & $config['smtp_host'].
Why know I have to start a STARTTLS, since rouncube is already on the mail server (with Postfix) and is an internal connection ? Or is the error message not completely correct ?
Can not 'smtp_host' just be localhost without tls:// ?
I didn't find the wiki what change between 1.4.x and 1.5.x.
thank you for some hint ;-)

[Dmitry42]

Try $config['smtp_host']='localhost:25'

[linpi]

thank's a lot, that was the answer.
The error message with STARTTLS was a bit confusing....