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I'm using zimbra on ubuntu as an email server behind a zentyal firewall. Im having problem Zimbra stopped receiving email from outside. Internal emails or outgoing is fine. ISP blocks all outgoing 25, but we are using their SMTP server. I have confirmed with ISP that they are not blocking incoming 25.

I have tested using smtp online test querying my server (hq.kiranamitra.co.id), errors with connection time out. If I change the port to 26, it can query my server no problem.

I run a tcpdump with below result:

12:03:05.750868 IP (tos 0x0, ttl 38, id 10558, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0x0563 (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005954406 ecr 0,nop,wscale 7], length 0

12:03:05.750912 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0xdaca), seq 3633868327, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19870434 ecr 4005954406], length 0

12:03:06.749941 IP (tos 0x0, ttl 38, id 11113, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0x017b (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005955406 ecr 0,nop,wscale 7], length 0

12:03:06.750013 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0xa29d), seq 3649478788, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19870684 ecr 4005955406], length 0

12:03:08.750232 IP (tos 0x0, ttl 38, id 12408, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0xf9aa (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005957406 ecr 0,nop,wscale 7], length 0

12:03:08.750278 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0xae9a), seq 3680733414, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19871184 ecr 4005957406], length 0

12:03:12.750834 IP (tos 0x0, ttl 38, id 15055, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0xea0a (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005961406 ecr 0,nop,wscale 7], length 0

12:03:12.750872 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0xc656), seq 3743242728, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19872184 ecr 4005961406], length 0

12:03:20.750664 IP (tos 0x0, ttl 38, id 21341, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0xcaca (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005969406 ecr 0,nop,wscale 7], length 0

12:03:20.750701 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0x4902), seq 3868240057, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19874184 ecr 4005969406], length 0

12:03:36.751320 IP (tos 0x0, ttl 38, id 31037, offset 0, flags [none], proto TCP (6), length 60)
    mail-wm0-f48.google.com.34872 > micro-mail.kma.lan.smtp: Flags [S], cksum 0x8c4a (correct), seq 701339940, win 42780, options [mss 1380,sackOK,TS val 4005985406 ecr 0,nop,wscale 7], length 0

12:03:36.751362 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > mail-wm0-f48.google.com.34872: Flags [S.], cksum 0x5e8b (incorrect -> 0x1124), seq 4118250384, ack 701339941, win 27360, options [mss 1380,sackOK,TS val 19878184 ecr 4005985406], length 0

12:07:06.433721 IP (tos 0x0, ttl 47, id 18497, offset 0, flags [DF], proto TCP (6), length 60)
    chrome-onfosi.cccampaigns.net.55094 > micro-mail.kma.lan.smtp: Flags [S], cksum 0x9bca (correct), seq 3650272210, win 14600, options [mss 1460,sackOK,TS val 214985990 ecr 0,nop,wscale 7], length 0

12:07:06.433762 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 56)
    micro-mail.kma.lan.smtp > chrome-onfosi.cccampaigns.net.55094: Flags [S.], cksum 0x8ccc (incorrect -> 0x7d31), seq 3827629515, ack 3650272211, win 27360, options [mss 1380,sackOK,TS val 19930605 ecr 214985990], length 0

It seems that outside MTA can connect to us (so ISP is not blocking?), but they are not receiving my server SIN ACK. And why all those checksum error for all outgoing? What is the problem with my server or my ISP?

Appreciate the help! THank you.

  • I can't tell who blocking what, but I can confirm that I can't establish a connection to port 25 on hq.kiranamitra.co.id (36.85.86.216) – HBruijn Sep 1 '16 at 5:47
  • Maybe your ISP is blocking outgoing packets with source port 25. – Law29 Sep 1 '16 at 6:45

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