Legitimate warning email from ITMC's SIC "Your credentials have appeared on the Internet"

the affected mail addresses (many old @uni-dortmund.de addresses), we suspect that the cause may lie far in the past. We have not received any further information from the DFN-Verein. However, the cause [...] The mail has been sent by the SIC of the ITMC to many TU members and is authentic. We often receive such warnings from the DFN-Verein (German Research Network), the TU's Internet provider. Based on the [...] cause is usually external web services that use the mail address as a user name and require a login with a password. If such a service is cracked by hackers and the data circulates in the darknet, DFN-Verein …