SMTPSendFailedException: 550 for server com.sun.mail.smtp that is not my configured SMTP Server

I have a workflow that reconciles time submitted on timesheets with vacation data from our payroll system and emails everyone who did not submit enough total time between the two of them during a given week.

This process has run for 2+ years including last Monday and today failed with the message:

ERROR Send Email 3:34 Execute failed: Error while communicating with the smtp server: com.sun.mail.smtp.SMTPSendFailedException: 550 5.7.607 Access denied, banned sending IP [my companies main domain]. To request removal from this list please visit and follow the directions. For more information please go to Remove yourself from the blocked senders list and address 5.7.511 Access denied errors - Office 365 | Microsoft Learn AS(1435) []

Note that “com.sun.mail.smtp” is NOT the SMTP server that I configured and am using in my workflow nodes.
Is this used by Knime under the hood?

Any advice on what is going on and why this might have started this week after 2+years of no issues.


More Details

My list of users that needed to receive email notifications had only 15 members. On the first run it send the email to 7 members. When I reran the process for the remaining members, it send out 3 emails and then again failed with the same spam/phishing error from Office 355.

My organization uses the same SMTP Server for other email notifications and never had issues.

It seems my email or domain is now listed as a SPAM site in Microsoft Office 365. I followed the instructions and link and could not be delisted. I had to click on an escalation request.

I am still able so send emails from the same FROM email from my personal outlook account and via Knime on the VM that I use.

Hi @mschutterop,
Thank you for reporting this. Could you please let us know which version of KNIME AP you are currently using?

Hi @mschutterop,

com.sun.mail.smtp is not some website or server on the internet, but the package name of the email client we use to send emails: com.sun.mail.smtp (JavaMail API documentation)
This is a very common client among Java applications.

I’m afraid the issue at hand is wholesome on the side of MS. Maybe some emails contained repetitive content that triggered some spam detection algorithm.

To resolve this, MS will need to step in. Sanket has a good point asking about the KNIME client version. We have updated the JavaMail library to a newer version in KNIME Analytics Platform 4.7. This change may have contributed to the classification on MS side, though it is still up to them to unblock you.

Kind regards


Thank you both for your responses. I am using knime 4.6.4 v202211210858. I use the SEND MAIL node in a TABLE ROW TO VARIABLE LOOP. Do you think upgrading to 4.7 with the newer Javamail library would help?

I upgraded to 4.7 and I haven’t had the issue in the recent weekly runs. So it was either transitory or the update helped. On the Microsoft front, you really don’t want to get in this state because the review process and escalation upon initial rejection leads nowhere with no further recourse. Thanks for the help and suggestions.



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