All outgoing MTAs are monitored using MXToolbox Blacklist Monitoring, which checks our sending IPs against a wide set of worldwide DNSBL/RBL providers.
The monitoring runs continuously and alerts whenever an IP becomes listed. If a listing occurs we handle delisting when possible. No customer action is required.

Below is a list of blocklists monitored through MXToolbox, including their active DNSBL endpoints where applicable.
Most of them are relevant for worldwide email delivery and are operationally reliable. They are transparent in listings and offer manual delisting procedures. 

Some DNSBLs (Abusix) do not expose endpoints publicly. MXToolbox supports them via API-level integration.

Microsoft does not appear in MXToolbox's monitored blacklist list because their blocklist is not publicly accessible. Instead of publishing listings, Microsoft only communicates blocks through SMTP bounce messages. Below is an example of such a bounce, which can indicate either an S3140 or S3150 block on their internal system. 

The mail system <xxx@hotmail.com>: host
hotmail-com.olc.protection.outlook.com[52.101.11.5] said: 550 5.7.1
Unfortunately, messages from [x.x.x.x] weren't sent. Please contact
your Internet service provider since part of their network is on our block
list (S3150). You can also refer your provider to
http://mail.live.com/mail/troubleshooting.aspx#errors.

This message confirms that Microsoft rejected the message due to a block on their internal IP reputation list. Unlike public DNSBLs, delisting must be handled through Microsoft Support and can't be queried directly online.

MultiRBL Valli: https://multirbl.valli.org
This tool checks a domain or IP against more than 200 DNS blacklists and whitelists, and also performs an FCrDNS identity validation. It provides a detailed list of results along with information about each blacklist, including any return codes associated with the listing.

There is no action required on the customer side. When a customer reports delivery problems caused by a blacklisting, Support first needs to determine whether the blacklist is part of our monitored list (as checked via MXToolbox monitoring).

No customer action is required. If one of our sending IP addresses is listed in a monitored blacklist, Open-Xchange will handle the delisting when possible. In most cases blacklist removal requests take 24-48 hours, depending on the blacklist provider.

Blacklist removals can take longer in cases where:
• The listing is automatically time-based (waiting period)
• The blacklist does not support manual removal
• The provider requires additional remediation steps

⛔ About paid delisting
Some blacklist providers, such as UCEPROTECT, offer faster removal in return for payment. Open-Xchange does not pay for delisting and we do not consider such lists trustworthy indicators of sender reputation. We also advice receivers not to use their blacklist service.

This requires manual investigation because it may not be a public block. The issue may be related to:
• Some providers do not provide API access for monitoring purposes
• Private or commercial blacklist, not publicly available (i.e. Microsoft)
• Single mail provider block block (policy based)
• Regional ISP block
• SPF, DMARC or DKIM failures

For these particular cases Open-Xchange needs to contact those providers and will handle the removal, if possible, or must wait until the IP address is automatically delisted.

Spamtrap-based listings
Certain blacklists (Lashback) list IPs when they receive mail addressed to spamtrap accounts. Because spamtrap addresses are intentionally hidden, neither we or the customer can identify them. We continuously improve detection of compromised accounts and suspicious senders to reduce these risks.