is there anything in /var/log/mail.log from that time-frame - (especially lines from pmg-smtp-filter would be interesting)
mail.log
Apr 6 10:51:34 mailgw postfix/postscreen[20264]: CONNECT from [37.140.190.187]:36655 to [192.168.1.20]:25
Apr 6 10:51:34 mailgw postfix/postscreen[20264]: warning: postscreen_access_list: non-null host address bits in "192.168.1.1/20", perhaps you should use "192.168.0.0/20" instead
Apr 6 10:51:34 mailgw postfix/postscreen[20264]: warning: postscreen_access_list: permit_mynetworks: mynetworks lookup error -- ignoring the remainder of this access list
Apr 6 10:51:34 mailgw postfix/postscreen[20264]: PASS OLD [37.140.190.187]:36655
Apr 6 10:51:34 mailgw postfix/smtpd[27063]: connect from forward106o.mail.yandex.net[37.140.190.187]
Apr 6 10:51:34 mailgw postfix/smtpd[27063]: DC9BB1006F3: client=forward106o.mail.yandex.net[37.140.190.187]
Apr 6 10:51:34 mailgw postfix/cleanup[27125]: DC9BB1006F3: message-id=<
445721617695471@mail.yandex.ru>
Apr 6 10:51:34 mailgw postfix/qmgr[11394]: DC9BB1006F3: from=<
user@example.com>, size=6049, nrcpt=1 (queue active)
Apr 6 10:51:34 mailgw pmg-smtp-filter[27055]: 2021/04/06-10:51:34 CONNECT TCP Peer: "[127.0.0.1]:39334" Local: "[127.0.0.1]:10024"
Apr 6 10:51:34 mailgw postfix/smtpd[27063]: disconnect from forward106o.mail.yandex.net[37.140.190.187] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
Apr 6 10:51:34 mailgw pmg-smtp-filter[27055]: 100700606C1306E19D4: new mail message-id=<
445721617695471@mail.yandex.ru>#012
Apr 6 10:51:35 mailgw pmg-smtp-filter[27055]: 100700606C1306E19D4: SA score=0/5 time=0.712 bayes=0.00 autolearn=no autolearn_force=no hits=AWL(0.725),BAYES_00(-1.9),HTML_MESSAGE(0.001),HTML_MIME_NO_HTML_TAG(0.377),KAM_DMARC_STATUS(0.01),KAM_LAZY_DOMAIN_SECURITY(1),MIME_HTML_ONLY(0.1),RCVD_IN_DNSWL_LOW(-0.7),RCVD_IN_MSPIKE_H2(-0.001),SPF_HELO_NONE(0.001),SPF_NONE(0.001),URIBL_BLOCKED(0.001)
Apr 6 10:51:35 mailgw pmg-smtp-filter[27055]: 100700606C1306E19D4: block mail to <
info@example.com> (rule: Blacklist)
Apr 6 10:51:35 mailgw pmg-smtp-filter[27055]: 100700606C1306E19D4: processing time: 0.736 seconds (0.712, 0.014, 0)
Apr 6 10:51:35 mailgw postfix/lmtp[27126]: DC9BB1006F3: to=<
info@example.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.77, delays=0.03/0/0/0.74, dsn=5.7.1, status=bounced (host 127.0.0.1[127.0.0.1] said: 554 5.7.1 Rejected for policy reasons (100700606C1306E19D4) (in reply to end of DATA command))
Apr 6 10:51:35 mailgw postfix/cleanup[27125]: A4649100765: message-id=<
20210406075135.A4649100765@mailgwexample.com>