25 SMTPSimple Mail Transfer Protocol for server-to-server email routing.
Open between mail relays. Often blocked outbound on client networks to curb spam.
| Transport | Status | Meaning |
|---|---|---|
| TCP | Yes | Assigned by IANA and standardized, specified, or widely used on this port. |
| UDP | Assigned | Assigned by IANA, but not standardized, specified, or widely used on this port. |
| SCTP | — | Not applicable for this transport. |
| DCCP | — | Not applicable for this transport. |
Well-known (0–1023). System port for a core service. On Unix-like systems, binding it requires superuser privileges.
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Copy-ready inbound rules for the active transports. Replace every <source> placeholder with the specific host, subnet, or CIDR that should reach this port.
sudo ufw allow proto tcp from <source> to any port 25Allow TCP 25 (SMTP). Replace <source> with the network/host that should reach it.
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="<source>" port port="25" protocol="tcp" accept'Allow TCP 25 (SMTP) from <source>. Run 'firewall-cmd --reload' afterwards.
nft add rule inet filter input ip saddr <source> tcp dport 25 acceptAllow TCP 25 (SMTP). Assumes table 'inet filter' with an 'input' chain.
sudo iptables -A INPUT -p tcp -s <source> --dport 25 -j ACCEPTAllow TCP 25 (SMTP). Persist with iptables-save/netfilter-persistent.
az network nsg rule create -g <resource-group> --nsg-name <nsg> -n Allow-smtp-25 --priority 1000 --access Allow --direction Inbound --protocol TCP --source-address-prefixes <source> --destination-port-ranges 25Inbound rule for 25 (SMTP). Set <source> to a CIDR; lower priority number = higher precedence.
aws ec2 authorize-security-group-ingress --group-id <sg-id> --ip-permissions IpProtocol=tcp,FromPort=25,ToPort=25,IpRanges="[{CidrIp=<source-cidr>}]"Inbound TCP 25 (SMTP). Replace <source-cidr> (e.g. 10.0.0.0/16).
gcloud compute firewall-rules create allow-smtp-25 --direction=INGRESS --action=ALLOW --rules=tcp:25 --source-ranges=<source-cidr> --network=<network>Ingress rule for 25 (SMTP). Scope with --source-ranges and optionally --target-tags.
netsh advfirewall firewall add rule name="Allow SMTP 25/TCP" dir=in action=allow protocol=TCP localport=25 remoteip=<source>Inbound TCP 25 (SMTP). Set remoteip to a host/subnet; 'any' is discouraged.
New-NetFirewallRule -DisplayName "Allow SMTP 25/TCP" -Direction Inbound -Action Allow -Protocol TCP -LocalPort 25 -RemoteAddress <source>Inbound TCP 25 (SMTP). Replace <source> with a host or subnet.
Rules are generated only for transports marked active for this port. They default to an inbound allow with an explicit source placeholder — never any. Review direction, scope, and rule precedence before applying in production.