Looks like Prolog is blocking a Google Gmail server today:
"Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 12): 553 sorry, your mailserver [18.104.22.168] is rejected by See »www.ptd.net/RBL."
Too bad I'm not the "responsible person" of the server in question as their web page states, so I can't report it to Prolog and help them out
I'm not wasting my time complaining to Google as any email server from google.com should be a no-brainer white listing in Prolog's private RBL.