in reply to Tio

UCEPROTECTL3 167.86.72.173 was listed <-- the server is listed

Connecting to 167.86.72.173

220 tromland.nohost.me Service ready [258 ms]
EHLO keeper-us-east-1c.mxtoolbox.com
250-tromland.nohost.me
250-PIPELINING
250-SIZE 35914708
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 CHUNKING [210 ms]
MAIL FROM:supertool@mxtoolboxsmtpdiag.com
250 2.1.0 Ok [210 ms]
RCPT TO:test@mxtoolboxsmtpdiag.com
554 5.7.1 test@mxtoolboxsmtpdiag.com: Relay access denied [300 ms]

in reply to Tio

The ip address has a TTL of 2100 or so and that used to be a reason for rbls to block items. I would simply contact the owner of the blocklist and ask them what to do. If there is a lot of spam going out through that server then it will never be whitelisted.

There is one advantage, you can ask for a ptr record and setup your own mailserver. Do be aware that needs to be maintained too