Port 587 SMTP submission · TCP

What runs on port 587

SMTP submission: where mail clients and applications hand outgoing mail to their own server, with authentication, before the server relays it on port 25. It starts in plaintext and upgrades with STARTTLS; a correctly configured server refuses AUTH before STARTTLS. Using 587 instead of 25 for clients is what makes ISP port-25 blocking harmless.

Software that uses it: Postfix (submission) · Exim · Exchange · Gmail/Outlook/SES/Mailgun SMTP endpoints · msmtp, swaks (clients)

Should port 587 be open?

Designed to face the internet. This port exists to be reachable from anywhere, so the usual question is not whether to open it but how the service behind it is configured and patched.

• swaks --to [email protected] --server mail:587 --tls --auth is the fastest end-to-end test.
• Rate-limit and require auth; an open submission port is an open relay.

Check port 587

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :587'
sudo lsof -i :587

# from another machine: is it reachable?
nc -zv host 587
nmap -p 587 host
curl -v telnet://host:587      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 587

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 587 proto tcp
sudo ufw deny 587/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 587 accept
nft add rule inet filter input tcp dport 587 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 587 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 587 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=587 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 587" with no source is just "open".

Related ports

• 465 SMTPS — Mail submission over implicit TLS
• 25 SMTP — Mail transfer between servers
• 143 IMAP — Mail access, plaintext/STARTTLS
• 993 IMAPS — IMAP over TLS

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions