A family member reported that she hadn't received an email message sent to
her today by a
Gmail user who had sent her message in reply to the family member's
email to her. Since I administer the
Sendmail
email server she uses, I checked the Sendmail log file at
/var/log/maillog. I saw the outgoing email sent to the Gmail
address, but no incoming email from that address. So I sent email
messages from a Gmail account I have as well as email messages from other
external addresses to the root account on the server. The other email
messages arrived, but none I sent from the Gmail account arrived. So I ran
tcpdump
on the server to capture data to/from port 25 on the system, which is the
well-known port for
Simple Mail Transfer Protocol (SMTP) traffic. I then sent
another email message to the root account on the Sendmail server from my
Gmail account. After allowing several minutes for an attempted delivery from
the Gmail server to occur, I stopped the packet capture with Ctrl-C.
# tcpdump -i enp1s4 port 25 -w smtp_2017-04-30.pcap tcpdump: listening on enp1s4, link-type EN10MB (Ethernet), capture size 65535 bytes ^C225 packets captured 225 packets received by filter 0 packets dropped by kernel # ls -lh smtp_2017-04-30.pcap -rw-r--r--. 1 tcpdump tcpdump 33K Apr 30 12:33 smtp_2017-04-30.pcap #
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