HMI panel email send error: check DNS first

Why checked SMTP addresses still fail

Retyping correct addresses cannot fix name resolution. A panel sends in four stages: it resolves the SMTP hostname to an IP address, opens a connection to the mail server, authenticates the sending account, and hands over the message. Resolving the hostname needs DNS, and the DNS settings live in the panel's own network settings, not in its email setup. A panel with blank DNS fields stops at stage 1. It never reaches the server, so the login details you keep re-checking are never tested. The error points at the stage where the send stopped, and this fault stops at the first stage.

Work from the network inward when a panel misbehaves. The switch in the path gets checked before the panel, and the same order settles a panel project download that fails through the network switch. The habit repeats across panel faults; the power pin on a programming cable gets the same treatment in check the HMI programming cable power pin first.

What each symptom points to

Match what the panel shows to a stage, then work on that stage:

• An error the instant you press send. The send stopped at name resolution. Go to the DNS tab.
• A send that hangs for 60 seconds or more, then times out. The panel got past the name but cannot reach the server. Look at routing, the mail port, and any firewall between the panel and the server.
• An authentication or login error. Resolution and connection now work. The SMTP account is wrong, disabled, or the server rejects the panel's authentication method.
• The send reports success and the inbox stays empty. The server accepted the message and a filter quarantined it afterward. That is a mail policy question, not a panel setting.

The first line is the common one, and it is the stage that three rounds of address checking never touch.

Enter the DNS server addresses first

1. Get the site's primary and alternate DNS server addresses from the network administrator. The addresses the office PC already uses make a good cross-check; read them off that PC's network settings and confirm them.
2. On the panel, open the network settings and find the DNS tab. Enter the primary DNS server address, then the alternate.
3. Save the settings and send a test message.

Use the addresses the site approves. On a plant network the administrator decides which resolvers a panel may query, and an address pasted in from memory can break reachability and site policy together.

Set the SMTP fields and leave POP3 blank

The email tab takes four entries:

• SMTP server. The outgoing mail server's hostname, such as mail.mail.com. Use the exact name the mail administrator gives you.
• POP3 account. Leave it blank. POP3 retrieves incoming mail; a panel that only sends never uses it.
• SMTP account email address. The sending account's own address.
• SMTP account login. That account's username and password. This is what satisfies an outgoing server that requires authentication, and it belongs here, not in a POP3 field.

Apply the settings and send a test message to an internal recipient. Note the time you sent it, so the administrator can search the server log for that minute.

Example: a 15-inch panel that fails with correct addresses

A 15-inch touchscreen in a plant office emails shift reports to a supervisor. The outgoing server requires authentication, and the panel reports a send error the moment a message is triggered, with the addresses checked more than once.

Inputs, and where each comes from:

• SMTP hostname: mail.mail.com, the outgoing name the mail administrator supplies.
• DNS fields on the panel: blank, read on the panel's network DNS tab.
• POP3 account: unused, because the panel only sends.
• SMTP account: one address with its login, confirmed active by the mail administrator.

The rule in words: a panel sends in four stages, and stage 1 turns the SMTP hostname into an IP address using DNS addresses held in the panel itself. Two addresses, primary and alternate, belong on the panel's DNS tab.

The arithmetic: the email tab takes 4 entries, and the working send-only setup fills 3 and leaves 1 blank, so 3 + 1 = 4. The send stops at stage 1 of 4, so stages 2, 3 and 4 run 0 times, which is why the 3 filled entries were never the problem. After the 2 DNS addresses go in, stage 1 resolves, and the later stages test settings that were already right.

Result: the 14:05 test message sends, the administrator finds it in the server log within 5 minutes of the send, and the report arrives in the supervisor's mailbox.

Decision: when a panel's two DNS fields are blank, DNS is the failing stage, and it is the first thing to fix. Chase credentials only when a retry fails with an authentication message, because that message means stages 1 and 2 passed.

If the send still fails

The send now moves far enough to show the next stage.

• Authentication fails. Ask the mail administrator to confirm the account is active, that SMTP authentication is enabled on it, and which connection the server expects. Mail servers accept the plain transfer port 25, the authenticated submission port 587, or a TLS-wrapped connection, and the panel has to support the one your site uses. If the panel's email feature cannot use that method, no password change restores the send, and the administrator chooses a relay the panel can reach.
• The send reports success and the message never arrives. Send one plain test message, text only, and ask the administrator to check the server's log and quarantine for the recipient at the time you sent it. Message content can trigger a filter after a clean send, so the stage to fix is mail policy, and the panel settings are finished.

Record the panel's existing email and DNS settings before editing anything, so the previous configuration can be restored. A temporary relay goes in only with the administrator's approval and its own credentials.

Recommendation

Recommendation: When a panel's email fails with the SMTP and login details confirmed correct, enter the site's primary and alternate DNS server addresses first, set the SMTP server name and the SMTP account's own address and login, leave the POP3 account blank for a send-only panel, and send a test message. Go back to the mail administrator only when the retry fails with an authentication message or a successful send never arrives; at that point the fault sits on the server side.

Alarm email and a screen at the line are different jobs. If the cell also needs standing notices or a status board that plays its content offline, that is a separate display, and our shop-floor and andon screens are built for it.

Questions