SMTP connection fails

[cherbst]

I am trying to use this handler with a sensu cluster and our E-Mail provider https://www.jpberlin.de/ using smtp.
When I am trying to send an E-Mail directly from one of the sensu backend machines like this:

sensuctl event info entitiy keepalive --format=json|./sensu-email-handler -f sensu@domain.de -t recipient@domain.de -s mail.jpberlin.de -u user@domain.de -p <redacted>

I get:

Error executing sensu-email-handler: error executing handler: 535 5.7.8 Error: authentication failed:

We can send E-Mails using smtp using this provider from other applications without problems.

JPBerlin states that they support STARTTLS and both ports 25 and 587, I tried both ports with the same result:

Zugang per TLS/STARTTLS
IMAP: 143
POP3: 110
SMTP: 25 bzw. 587

https://www.jpberlin.de/hilfe/e-mail/wie-kann-ich-meine-e-mails-mit-meinem-mailprogramm-abrufen/

I am pretty much out of ideas here. Any help would be appreciated.