Postfix服务器间歇性外部无法访问（本地Roundcube正常）排查求助

各位大佬好，我昨天刚搭建好Postfix邮件系统，一开始一切正常，也能正常访问。但今天出现了好几次间歇性无法访问的情况，我完全摸不着头脑，想请大家帮忙分析下问题。

具体情况是这样的：当我在电脑上用Mozilla Thunderbird登录邮箱后，再尝试在手机上登录同一个邮箱账号时，手机端的配置一直失败。之后就连服务器的IMAP和SMTP服务都完全没响应了——我用telnet测试连接也连不上。但奇怪的是，等个几分钟之后，服务器又突然恢复正常，跟什么都没发生过一样。

有个关键点：服务器本地用Roundcube登录一直是正常的，完全不受影响。

我查了日志，没看到任何异常记录，实在找不到问题出在哪。下面是我的main.cf配置文件内容：

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no
# appending .domain is the MUA's job.
append_dot_mydomain = no
# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h
readme_directory = no
# See http://www.postfix.org/COMPATIBILITY_README.html -- default to 2 on
# fresh installs.
compatibility_level = 2
# TLS parameters
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_security_level = may
smtp_tls_CApath=/etc/ssl/certs
smtp_tls_security_level = dane
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = example.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, , localhost
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
mailbox_size_limit = 0
recipient_delimiter = +
inet_protocols = all
virtual_alias_maps = hash:/etc/postfix/virtual
sender_bcc_maps = hash:/etc/postfix/bcc
sender_dependent_default_transport_maps = hash:/etc/postfix/dependent
home_mailbox = Maildir/
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
smtp_dns_support_level = dnssec
smtp_host_lookup = dns
allow_percent_hack = no
tls_server_sni_maps = hash:/etc/postfix/sni_map
mynetworks_style = subnet

真心希望各位能给点排查思路或者解决方案，万分感谢！

备注：内容来源于stack exchange，提问作者sebastian307