ControlForge SMTP Email Guide

James M. Belcher Founder, JMB Technical Services LLC April 2026 | ControlForge v1.0.535

1. Overview

ControlForge provides 4 built-in functions for sending email from Structured Text. Send alarm notifications, shift reports, and diagnostic alerts directly from your PLC program — no external scripts or middleware required.

Function	Auth	TLS	Port	Body Format
`SMTP_SEND`	None (relay)	No	25	Plain text
`SMTP_SEND_HTML`	None (relay)	No	25	HTML
`SMTP_SEND_AUTH`	Username/password	No	587	Plain text
`SMTP_SEND_TLS`	Username/password	Implicit TLS	465	Plain text

2. SMTP_SEND — Plain Relay

No authentication — sends through an open relay or local mail server. Typical for internal plant networks with a local SMTP relay.

ok := SMTP_SEND(
    'mail.plant.local:25',           (* SMTP server *)
    'plc@plant.local',               (* From *)
    'operator@plant.local',          (* To *)
    'High Temperature Alarm',        (* Subject *)
    'Temperature exceeded 180F at Zone 3. Check cooling system.'
);

Param	Type	Description
`host`	STRING	SMTP server address (host:port, default port 25)
`from`	STRING	Sender email address
`to`	STRING	Recipient email address
`subject`	STRING	Email subject line
`body`	STRING	Plain text body

Returns: BOOL — TRUE if the email was accepted by the server.

3. SMTP_SEND_HTML — HTML Email

Same as SMTP_SEND but sends HTML content. No authentication.

html := CONCAT(
    '<h2 style="color:red">ALARM: High Temperature</h2>',
    '<table border="1">',
    '<tr><td>Zone</td><td>3</td></tr>',
    '<tr><td>Temperature</td><td>', REAL_TO_STRING(temperature), ' F</td></tr>',
    '<tr><td>Threshold</td><td>180 F</td></tr>',
    '<tr><td>Time</td><td>', DT_TO_STRING(NOW()), '</td></tr>',
    '</table>',
    '<p>Please investigate immediately.</p>'
);

ok := SMTP_SEND_HTML(
    'mail.plant.local:25',
    'plc@plant.local',
    'operator@plant.local',
    'ALARM: High Temperature - Zone 3',
    html
);

4. SMTP_SEND_AUTH — Authenticated

Uses PLAIN authentication on port 587. Required for most external mail providers.

ok := SMTP_SEND_AUTH(
    'smtp.gmail.com:587',            (* SMTP server *)
    'plc.alerts@gmail.com',          (* Username *)
    'app-password-here',             (* Password / app password *)
    'plc.alerts@gmail.com',          (* From *)
    'maintenance@company.com',       (* To *)
    'ControlForge Daily Report',            (* Subject *)
    report_text                      (* Body *)
);

Param	Type	Description
`host`	STRING	SMTP server (host:port, default port 587)
`username`	STRING	Auth username
`password`	STRING	Auth password or app password
`from`	STRING	Sender address
`to`	STRING	Recipient address
`subject`	STRING	Subject line
`body`	STRING	Plain text body

5. SMTP_SEND_TLS — Encrypted (Implicit TLS)

Uses implicit TLS on port 465 — the connection is encrypted from the start. TLS 1.2 minimum.

ok := SMTP_SEND_TLS(
    'smtp.office365.com:465',
    'plc-alerts@company.com',
    'password',
    'plc-alerts@company.com',
    'supervisor@company.com',
    'Critical Fault - Line 2 Stopped',
    CONCAT('Line 2 stopped at ', DT_TO_STRING(NOW()),
           '. Fault code: ', INT_TO_STRING(fault_code))
);

Same parameters as SMTP_SEND_AUTH. Connection uses tls.Dial — not STARTTLS.

6. Complete Example: Alarm Notification System

PROGRAM POU_AlarmEmail
VAR
    initialized : BOOL := FALSE;
    scan_count : DINT := 0;

    (* Alarm states *)
    high_temp : BOOL;
    low_pressure : BOOL;
    e_stop : BOOL;

    (* Edge detection — send once per alarm *)
    high_temp_sent : BOOL := FALSE;
    low_press_sent : BOOL := FALSE;
    e_stop_sent : BOOL := FALSE;

    (* Throttle — one email per 5 minutes max *)
    email_throttle : STRING;
    ok : BOOL;
    body : STRING;

    (* Config *)
    smtp_host : STRING := 'smtp.company.com:587';
    smtp_user : STRING := 'plc-alerts@company.com';
    smtp_pass : STRING := 'app-password';
    from_addr : STRING := 'plc-alerts@company.com';
    to_addr : STRING := 'operators@company.com';
END_VAR

IF NOT initialized THEN
    email_throttle := THROTTLE_CREATE(300000);    (* 5 minute minimum between emails *)
    initialized := TRUE;
END_IF;

(* High temperature alarm *)
IF high_temp AND NOT high_temp_sent AND THROTTLE_ALLOW(email_throttle) THEN
    body := CONCAT(
        'HIGH TEMPERATURE ALARM', CHR(10), CHR(10),
        'Temperature: ', REAL_TO_STRING(temperature), ' F', CHR(10),
        'Threshold: 180 F', CHR(10),
        'Time: ', DT_TO_STRING(NOW()), CHR(10),
        'Location: Zone 3', CHR(10), CHR(10),
        'Please investigate immediately.'
    );

    ok := SMTP_SEND_AUTH(smtp_host, smtp_user, smtp_pass,
                         from_addr, to_addr,
                         'ALARM: High Temperature',
                         body);
    IF ok THEN high_temp_sent := TRUE; END_IF;
END_IF;

IF NOT high_temp THEN high_temp_sent := FALSE; END_IF;

(* E-stop alarm — always send, override throttle *)
IF e_stop AND NOT e_stop_sent THEN
    ok := SMTP_SEND_AUTH(smtp_host, smtp_user, smtp_pass,
                         from_addr, to_addr,
                         'CRITICAL: Emergency Stop Activated',
                         CONCAT('E-Stop activated at ', DT_TO_STRING(NOW())));
    IF ok THEN e_stop_sent := TRUE; END_IF;
END_IF;

IF NOT e_stop THEN e_stop_sent := FALSE; END_IF;

END_PROGRAM

7. Complete Example: Shift Report

PROGRAM POU_ShiftReport
VAR
    state : INT := 0;
    scan_count : DINT := 0;
    report_interval : DINT := 288000;   (* Every 8 hours at 100ms scan *)
    ok : BOOL;
    report : STRING;
    total_parts : DINT;
    total_faults : INT;
    avg_temp : REAL;
    uptime_hrs : REAL;
END_VAR

scan_count := scan_count + 1;

IF (scan_count MOD report_interval) = 0 THEN
    report := CONCAT(
        'SHIFT REPORT - ', DT_TO_STRING(NOW()), CHR(10),
        '========================================', CHR(10),
        'Parts Produced: ', DINT_TO_STRING(total_parts), CHR(10),
        'Fault Count:    ', INT_TO_STRING(total_faults), CHR(10),
        'Avg Temp:       ', REAL_TO_STRING(avg_temp), ' F', CHR(10),
        'Uptime:         ', REAL_TO_STRING(uptime_hrs), ' hours', CHR(10),
        '========================================', CHR(10),
        'ControlForge Automated Report'
    );

    ok := SMTP_SEND_AUTH(
        'smtp.company.com:587',
        'plc-reports@company.com', 'app-password',
        'plc-reports@company.com',
        'production-team@company.com',
        CONCAT('Shift Report - ', DT_TO_STRING(NOW())),
        report
    );

    (* Reset shift counters *)
    IF ok THEN
        total_parts := 0;
        total_faults := 0;
    END_IF;
END_IF;
END_PROGRAM

8. Tips

Use app passwords for Gmail, Office 365, etc. — regular passwords won't work with 2FA enabled.
Use SMTP_SEND for internal relay — most plant networks have a local SMTP relay that doesn't require auth. Simpler and more reliable.
Use SMTP_SEND_TLS for external services — encrypts credentials and content in transit.
Throttle alarm emails — use THROTTLE_CREATE from the resilience library to prevent email storms.
Edge detect alarms — send once when alarm activates, not every scan cycle.
Include context — timestamp, tag values, thresholds, and location in every alarm email.

Appendix A: Quick Reference

Function	Parameters	Returns	Description
`SMTP_SEND(host, from, to, subject, body)`	5	BOOL	Plain relay, no auth, port 25
`SMTP_SEND_HTML(host, from, to, subject, html)`	5	BOOL	HTML body, no auth, port 25
`SMTP_SEND_AUTH(host, user, pass, from, to, subject, body)`	7	BOOL	PLAIN auth, port 587
`SMTP_SEND_TLS(host, user, pass, from, to, subject, body)`	7	BOOL	Implicit TLS, port 465

ControlForge v1.0.535 | 4 SMTP Functions | Email Alerts from ST

1. Overview​

2. SMTP_SEND — Plain Relay​

3. SMTP_SEND_HTML — HTML Email​

4. SMTP_SEND_AUTH — Authenticated​

5. SMTP_SEND_TLS — Encrypted (Implicit TLS)​

6. Complete Example: Alarm Notification System​

7. Complete Example: Shift Report​

8. Tips​

Appendix A: Quick Reference​