The port 25/TCP…. is used by…SMTP The main purpose of this system is to transport messages between mail servers. It does not correspond, in a modern architecture, to the usual authenticated submission port of users; its exposure and filtering must therefore be assessed according to the actual role of the server….
What’s Port 25 for?….
The port 25/TCP…. used mainly for…SMTP transport between mail servers….. A server that receives messages for a domain may therefore need to be connected to 25 from the Internet. This role is different from authenticated submission of a user or,an application, often performed on dedicated ports such as 587 or 465 depending on the service….
It is necessary to distinguish SMTP connectivity, TLS negotiation, relay policy and final decision of the server on the message…
Transport Protocol
TCP
Safety precautions
Should we open this port in the firewall?
An Internet exposure of port 25 may be normal for an MX server, but it must be accompanied by a strict relay configuration: the server must not become a server…. open relay…. Limit authenticated submission to the mechanisms provided by your platform, activate TLS when available and monitor releases, files of wait and abnormal behaviors….
How do I check this port?
Start by testing the, establishment with the…Test-NetConnection SERVER -Port 25. To scan SMTP itself, check the banner, send a…. EHLO and check the announced extensions. If ▼TLS is proposed,…. openssl s_client -starttls smtp -connect SERVER:25 allows for the inspection of TLS trading….
- Identify which server should accept the message and for which domain….
- Read the exact SMTP code returned, including 4xx or 5xx….
- Check DNS/MX, TLS and send/recipient policy according to symptom….
- Only after having separated network transport and SMTP application refusal, will it be concluded that there is a port problem….
FAQ — port 25 / SMTP….
Should port 25 still be closed on the Internet?….
No. A server that receives mail directly from a domain may have to accept SMTP on port 25. The decision depends on the actual role of the server….
Why use 587 instead of 25 for an email client?…
Port 587 is commonly used for authenticated submission of messages by users or applications, while 25 remain oriented transport between servers….
Does a successful test of tc guarantee the sending mail?….
No. The server can accept the connection and then refuse the message for authentication, relay, recipient, reputation, policy or other SMTP code….