Mail cannot send to sophos customers, Client host [xx.xx.xx.xx] is blacklisted...

Is this happening to multiple recipients that utilize SOPHOS MTA?

yes, multiple recipient

May 18 16:54:19 magic-gra1 postfix/smtp[11738]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181]:25, delay=0.99, delays=0.08/0/0.76/0.15, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 18 16:57:27 magic-gra1 postfix/smtp[10297]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.59.152.81]:25, delay=0.71, delays=0.06/0/0.5/0.15, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.59.152.81] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 18 17:17:54 magic-gra1 postfix/smtp[18842]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181]:25, delay=0.65, delays=0.24/0/0.27/0.14, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 18 17:25:37 magic-gra1 postfix/smtp[20268]: ..., relay=mx-02-eu-central-1.prod.hydra.sophos.com[52.58.154.181]:25, delay=0.68, delays=0.22/0/0.31/0.15, dsn=5.7.1, status=bounced (host mx-02-eu-central-1.prod.hydra.sophos.com[52.58.154.181] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 18 18:27:52 magic-gra1 postfix/smtp[6267]: ..., relay=mx-01-eu-west-1.prod.hydra.sophos.com[34.250.76.189]:25, delay=0.95, delays=0.22/0/0.69/0.04, dsn=5.7.1, status=bounced (host mx-01-eu-west-1.prod.hydra.sophos.com[34.250.76.189] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 19 09:13:56 magic-gra1 postfix/smtp[8041]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.59.152.81]:25, delay=0.63, delays=0.15/0/0.34/0.15, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.59.152.81] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 19 09:14:12 magic-gra1 postfix/smtp[8024]: ..., relay=mx-01-eu-west-1.prod.hydra.sophos.com[52.30.72.188]:25, delay=0.68, delays=0.23/0/0.4/0.05, dsn=5.7.1, status=bounced (host mx-01-eu-west-1.prod.hydra.sophos.com[52.30.72.188] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 19 09:37:24 magic-gra1 postfix/smtp[14129]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181]:25, delay=0.54, delays=0.1/0/0.3/0.14, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))
May 19 10:32:31 magic-gra1 postfix/smtp[823]: ..., relay=mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181]:25, delay=0.96, delays=0.21/0/0.6/0.15, dsn=5.7.1, status=bounced (host mx-01-eu-central-1.prod.hydra.sophos.com[52.58.154.181] said: 554 5.7.1 Service unavailable; Client host [91.121.253.253] is blacklisted. Visit www.sophos.com/.../ip-lookup.aspx to request delisting (in reply to RCPT TO command))

postfix log
https://pastebin.com/cyMBkUvh

Interesting, Sophos Labs clearly shows it is not blocked (on their threat list). 

Hopefully someone from Support can chime in. The link to dispute an IP seems to go in loops...

the problem is that seem that sophos does not offer the minimal contact if you are not a sophos customer.. 
via online chat says me to ask the recipient to open a case but the recipient is not clearly a sysadmin

In this case, I would think the owner/admin of the IP address should have a way to request removal from the blacklist. I know this was an option but I can't find the URL. FloSupport can you offer any assistance on where this can be done?

Hello Marco,

Thank you for contacting the Sophos Community.

If you already confirm that the domain and IP being used isn’t blacklisted in any blacklist, send me a PM and I can open a ticket in your behalf with Labs to whitelist it.

I would need to domain and Public IP of the affected sender, also make sure they are not in any blacklist.

Regards,