Persistent IP reputation issues: debugging unexpected blacklisting of new server range's IPs

hey everyone, i'm realy hitting a wall with some persistent IP reputation issues after a recent infrastructure migration. we just moved our primary mail server, running Postfix and Dovecot, to a newly acquired /24 IP block. our old IPs had a spotless reputation for years, never an issue, so this is quite frustrating. almost immediately after the switch, we started seeing a significant increase in bounce rates for legitimate emails. turns out, several IPs within this new range are already on various blacklists, which is just baffling because we haven't done any bulk sending or questionable activities. our IP reputation is taking a serious hit, and it's affecting our core business communications.

• verified all our DNS records (PTR, SPF, DKIM, DMARC are all correctly configured).
• scanned outgoing mail queues for any anomalies or signs of compromise, but everything looks clean.
• used tools like MXToolbox, SenderScore, and TalosIntelligence, all confirm the bad IP reputation for these specific IPs.
• also double-checked for open relays and compromised accounts, but found none.

what's really throwing me off is the lack of a clear cause. this isn't just one or two rejections; it's a consistent pattern. for example, here's a typical rejection log we're seeing:

Jan 15 10:35:22 mailserver postfix/smtp[12345]: 1234ABCD: to=<[email protected]>, relay=mx.example.com[192.0.2.1]:25, delay=0.5, delays=0.1/0.01/0.2/0.19, dsn=5.7.1, status=bounced (host mx.example.com[192.0.2.1] said: 550 5.7.1 Service unavailable; Client host [203.0.113.50] blocked using zen.spamhaus.org; https://www.spamhaus.org/sbl/query/SBLXXXXXX (in reply to RCPT TO command))

i'm kinda stumped. are there any more advanced network diagnostics i should be running? could there be some lingering historical bad reputation attached to this /24 block that we're inheriting, even if the previous owner wasn't us? what specific server logs, beyond mail logs, should i be monitoring more closely for clues? realy looking for some expert insights on how to tackle this deep-seated IP reputation problem. thanks in advance for any help.