This listing occurs on the IP level. Delivery will more than likely be rejected and can also cause deliverability issues.
To better understand the potential problems that caused the listing, review the SBL website. Once you have reviewed, you should then:
- Find and correct the root cause of the listing.
- Contact Mailgun and provide details on root cause and what was done to resolve. Mailgun will then start the removal request.
It is important that the issue be resolved prior to contacting Mailgun. Being honest and transparent will get you to a solution faster. Spamhaus is straightforward on what they want to know. If Spamhaus detects that a problem is ongoing they will not remove the block.