Some droplet can't connect to smtp port.

Hello,

I encountered an issue on some Droplets where they cannot connect to other hosts using port 587 (SMTP protocol). I have checked the firewall group rules and iptables rules on the affected Droplets, but I did not find any rules blocking outgoing TCP port 587.

To debug the issue, I tested by setting up a listening TCP port 587 on another Droplet within the same network. When I tried to connect from the affected Droplet using telnet, it failed. However, when I reversed the test, the other Droplet could successfully connect back to TCP port 587 on the affected Droplet.

Additionally, I tested connecting to smtp.gmail.com on TCP port 587 and captured tcpdump traffic to compare between a working and a non-working Droplet. The results were as follows:

• Droplet that can connect to smtp.gmail.com:

tcpdump -i eth0 port 587
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
17:26:01.605785 IP dgo-ter-prd-ads-1.63174 > sa-in-f108.1e100.net.submission: Flags [S], seq 1518527726, win 42340, options [mss 1460,sackOK,TS val 2920465099 ecr 0,nop,wscale 9], length 0
17:26:01.610775 IP sa-in-f108.1e100.net.submission > dgo-ter-prd-ads-1.63174: Flags [S.], seq 716093568, ack 1518527727, win 65535, options [mss 1412,sackOK,TS val 1948154854 ecr 2920465099,nop,wscale 8], length 0
17:26:01.610823 IP dgo-ter-prd-ads-1.63174 > sa-in-f108.1e100.net.submission: Flags [.], ack 1, win 83, options [nop,nop,TS val 2920465104 ecr 1948154854], length 0
17:26:01.771273 IP sa-in-f108.1e100.net.submission > dgo-ter-prd-ads-1.63174: Flags [P.], seq 1:76, ack 1, win 1050, options [nop,nop,TS val 1948155015 ecr 2920465104], length 75
17:26:01.771344 IP dgo-ter-prd-ads-1.63174 > sa-in-f108.1e100.net.submission: Flags [.], ack 76, win 83, options [nop,nop,TS val 2920465265 ecr 1948155015], length 0
17:26:10.994375 IP dgo-ter-prd-ads-1.63174 > sa-in-f108.1e100.net.submission: Flags [P.], seq 1:3, ack 76, win 83, options [nop,nop,TS val 2920474488 ecr 1948155015], length 2
17:26:10.995834 IP sa-in-f108.1e100.net.submission > dgo-ter-prd-ads-1.63174: Flags [.], ack 3, win 1050, options [nop,nop,TS val 1948164240 ecr 2920474488], length 0
17:26:11.208083 IP dgo-ter-prd-ads-1.63174 > sa-in-f108.1e100.net.submission: Flags [P.], seq 3:5, ack 76, win 83, options [nop,nop,TS val 2920474702 ecr 1948164240], length 2

• Droplet that cannot connect to smtp.gmail.com:

tcpdump -i eth0 port 587
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
17:17:08.636918 IP dgo-ter-prd-red-1.9018 > sh-in-f108.1e100.net.submission: Flags [S], seq 1406560675, win 42340, options [mss 1460,sackOK,TS val 1272778776 ecr 0,nop,wscale 9], length 0
17:17:09.644587 IP dgo-ter-prd-red-1.9018 > sh-in-f108.1e100.net.submission: Flags [S], seq 1406560675, win 42340, options [mss 1460,sackOK,TS val 1272779784 ecr 0,nop,wscale 9], length 0
17:17:11.660629 IP dgo-ter-prd-red-1.9018 > sh-in-f108.1e100.net.submission: Flags [S], seq 1406560675, win 42340, options [mss 1460,sackOK,TS val 1272781800 ecr 0,nop,wscale 9], length 0

Could you help me investigate what might be causing this issue?