Is your IP at DigitalOcean one of the thousands that have been blacklisted?
Seems like a whole block of IP addresses at Digital Ocean have been blacklisted at Spamhaus:
A cut and paste of the Removal Procedure is below:
To have record SBL368922 (126.96.36.199/20) removed from the SBL, the Abuse/Security representative of digitalocean.com needs to contact the SBL Team by email to explain how the abuse problem has been terminated (we need to know exactly how the issue has been dealt with and that this abuse problem is fully terminated).
If the abuse problem that caused this listing has been terminated we will normally remove the listing from the SBL without delay.
Botnet hosting going on for weeks, ignoring abuse reports and multiple reminders sent by Spamhaus and 3rd parties: