Stop using ZapBL blocklist, as it is nothing but extortion
From what Office 365 Support says, emails are considered as spams by Outlook filters if the sending IP is in the ZapBL blocklist.
Many other DNSBL are far more trustworthy than ZapBL, like SpamHaus, SpamCop or Barracuda.
False positives with ZapBL are very frequent.
For example, they regularly blacklist the whole IP range 126.96.36.199/19 (8,192 IPs !) because of the single spammy IP 188.8.131.52.
You can check it via https://zapbl.net/lookup.php.
Consequently, all the neighbour IPs are regularly blacklisted at the same time, although they have nothing to do with this damned spammer !
Furthermore, if you want to get help from ZapBL, you have to pay first :
Thank you for your understanding.