自从最新的 yum 更新（几个月来的第一次更新）以来，POP3 已经停止工作。
我再次检查了 pop3 是否列在公共区域，果然如此：
firewall-cmd --list-all --zone=public
public (active)
target: default
icmp-block-inversion: no
interfaces: eth0
sources:
services: cockpit dhcpv6-client http https imap imaps pop3 smtp-submission smtps ssh
ports: 25/tcp 80/tcp
protocols:
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:
但是 netstat 没有显示端口 110 是开放的：
netstat -tulnp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:587 0.0.0.0:* LISTEN 2683/master
tcp 0 0 127.0.0.1:783 0.0.0.0:* LISTEN 863/perl
tcp 0 0 0.0.0.0:465 0.0.0.0:* LISTEN 2683/master
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 1355/sshd
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 2683/master
tcp 0 0 0.0.0.0:5666 0.0.0.0:* LISTEN 862/nrpe
tcp 0 0 127.0.0.1:10024 0.0.0.0:* LISTEN 2703/amavisd (maste
tcp 0 0 127.0.0.1:10025 0.0.0.0:* LISTEN 2683/master
tcp 0 0 127.0.0.1:10026 0.0.0.0:* LISTEN 2703/amavisd (maste
tcp6 0 0 :::587 :::* LISTEN 2683/master
tcp6 0 0 ::1:783 :::* LISTEN 863/perl
tcp6 0 0 :::80 :::* LISTEN 860/httpd
tcp6 0 0 :::465 :::* LISTEN 2683/master
tcp6 0 0 :::22 :::* LISTEN 1355/sshd
tcp6 0 0 :::25 :::* LISTEN 2683/master
tcp6 0 0 :::443 :::* LISTEN 860/httpd
tcp6 0 0 :::5666 :::* LISTEN 862/nrpe
tcp6 0 0 ::1:10024 :::* LISTEN 2703/amavisd (maste
tcp6 0 0 ::1:10026 :::* LISTEN 2703/amavisd (maste
udp 0 0 127.0.0.1:323 0.0.0.0:* 659/chronyd
udp6 0 0 ::1:323 :::* 659/chronyd
我已仔细检查过 POP3 的服务文件是否存在且包含端口 110，结果确实如此。
如果我监听端口 110 并 ping 的话，会显示以下内容（但该端口报告为已关闭）：
tcpdump -i any port 110
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
12:48:49.060706 IP 198.199.98.246.43347 > domain.net.pop3: Flags [S], seq 3072871086, win 14600, options [mss 1460,sackOK,TS val 3789040058 ecr 0,nop,wscale 8], length 0
12:48:49.060779 IP domain.net.pop3 > 198.199.98.246.43347: Flags [R.], seq 0, ack 3072871087, win 0, length 0
12:48:49.193808 IP 198.199.98.246.43350 > domain.net.pop3: Flags [S], seq 4058864357, win 14600, options [mss 1460,sackOK,TS val 3789040092 ecr 0,nop,wscale 8], length 0
12:48:49.193880 IP domain.net.pop3 > 198.199.98.246.43350: Flags [R.], seq 0, ack 4058864358, win 0, length 0
12:48:49.327749 IP 198.199.98.246.43351 > domain.net.pop3: Flags [S], seq 1901081854, win 14600, options [mss 1460,sackOK,TS val 3789040125 ecr 0,nop,wscale 8], length 0
12:48:49.327824 IP domain.net.pop3 > 198.199.98.246.43351: Flags [R.], seq 0, ack 1901081855, win 0, length 0
如果我检查邮件，会发现类似的转储，但 Dovecot 没有记录任何内容。