Their mail excerpt: This system has not sent any e-mail to our customers for a long time. For security reasons our systems will only accept e-mails from such IP addresses after a check of setup and information about these systems.
Please give us details about this system and the company using it, tell us all about the sending domain, what type of e-mail will be sent and especially if you or your customer want to send newsletter give us detailed information on how recipients e-mail addresses had been acquired. Who in person is responsible for e-mail sent from this system (MTA)?
Please be advised that only technically proper configured and very well maintained systems are qualified for a reset of reputation and please see our FAQ section 4.1 (Requirements for smooth access to our e-mail exchanges <https://postmaster.t-online.de/index.en.html#t4.1>):
"There must be a domain and website with direct contact information easily deducible from the delivering IP's hostname (FQDN)."