i made a test with mxtoolbox
i got this in my postfix logs
01/05/2021, 08:19:31 PM info statistics: max cache size 1 at Jan 5 20:15:30
01/05/2021, 08:19:31 PM info statistics: max connection count 1 for (smtpd:18.205.72.90) at Jan 5 20:15:30
01/05/2021, 08:19:31 PM info statistics: max connection rate 2/60s for (smtpd:18.205.72.90) at Jan 5 20:16:10
01/05/2021, 08:16:11 PM info disconnect from keeper-us-east-1c.mxtoolbox.com[18.205.72.90] ehlo=1 mail=1 rcpt=0/1 quit=1 commands=Âľ
01/05/2021, 08:16:10 PM info NOQUEUE: reject: RCPT from keeper-us-east-1c.mxtoolbox.com[18.205.72.90]: 554 5.7.1 test@mxtoolboxsmtpdiag.com: Relay access denied; from=supertool@mxtoolboxsmtpdiag.com to=test@mxtoolboxsmtpdiag.com proto=ESMTP helo=<keeper-us-east-1c.mxtoolbox.com>
01/05/2021, 08:16:10 PM info connect from keeper-us-east-1c.mxtoolbox.com[18.205.72.90]
01/05/2021, 08:16:10 PM info PASS OLD [18.205.72.90]:23361
01/05/2021, 08:16:10 PM info Look up 18.205.72.90 on whitelist, result 200 DUNNO
01/05/2021, 08:16:10 PM info CONNECT from [18.205.72.90]:23361 to [172.22.1.11]:25
01/05/2021, 08:15:33 PM info disconnect from keeper-us-east-1c.mxtoolbox.com[18.205.72.90] ehlo=1 mail=1 rcpt=0/1 quit=1 commands=Âľ
01/05/2021, 08:15:32 PM info NOQUEUE: reject: RCPT from keeper-us-east-1c.mxtoolbox.com[18.205.72.90]: 554 5.7.1 test@mxtoolboxsmtpdiag.com: Relay access denied; from=supertool@mxtoolboxsmtpdiag.com to=test@mxtoolboxsmtpdiag.com proto=ESMTP helo=<keeper-us-east-1c.mxtoolbox.com>