3CX and other phone systems with Microsoft 365

Phone systems email voicemails, faxes, call reports and alarms. 3CX, Yeastar, Grandstream UCM, FreePBX and most hosted-on-premises PBXs use plain SMTP with optional password authentication, which Microsoft 365 has retired. Relaying through the appliance keeps voicemail-to-email working without a mailbox password in the PBX.

What the device needs

SMTP server: the SMTP-365 appliance's IP address, on your LAN.
Port: 2525 (or 25 if the device cannot change its port).
Authentication: none. Encryption: none. The relay is plain SMTP on the LAN and does the secure part to Microsoft itself.
Sender address: a real mailbox in your tenant (a shared "alerts" mailbox is free). The appliance can override it so every alert comes from the same address.
The device's address or subnet must be on the appliance's devices list (Settings → Devices).

Where the settings are

3CX (V18 and V20)

Admin Console → Settings → Email (V18: Settings → Email): choose Custom SMTP server, SMTP server = the SMTP-365 appliance's IP address, Port = 2525, leave username and password blank, TLS/SSL off, Reply-to address = the scanner or notifications mailbox. Use Test. If your 3CX edition only offers the built-in 3CX mail service, leave it; nothing to change.

Yeastar P-Series / S-Series

System → Email: Custom SMTP, server, port, no authentication, no encryption.

Grandstream UCM

System Settings → Email Settings: Custom SMTP server, port, no authentication, TLS off.

FreePBX / Asterisk

System Admin → Email Setup, or postfix relayhost = [appliance-ip]:2525 with no SASL.

Voicemail attachments are ordinary email to the user's own mailbox, so they arrive as internal mail from the notifications address. If you would rather each voicemail appear to come from the PBX, leave the appliance's sender override off and allow the PBX's From address as a sender.

If the test message does not arrive

What you see	Likely cause	Fix
Connection failed / cannot connect to server	Wrong relay address or port, or the device's subnet is not in the relay's allowed sources	Check the address and port; on the appliance, Settings → Devices, add the device's subnet
554 5.7.1 Relaying denied (or the connection closes at once)	The device's IP is not among the appliance's devices	Add the address or subnet under Settings → Devices on the appliance
Authentication or certificate error on the device	Authentication or TLS is still switched on in the device	Set authentication to none/off and SSL/TLS to off; the relay is plain SMTP on the LAN
Device says sent, nothing arrives	Delivery to Microsoft 365 failed and the job is held or failed	Open the appliance's Status page: held and failed jobs show the reason and can be re-queued
Arrives, but from the wrong address	The device's own From address is being used	Turn on the sender override on the appliance so every message is sent from the scanner mailbox

One appliance for all of them

The same SMTP-365 appliance that fixes the printers takes the UPS, the servers' management cards, the NAS and the phone system, on one licence with no per-device fees. Each device gets the same three settings and the appliance's Status page shows every message it handled. Background: what Microsoft changed.

Fix it once, for every device. SMTP-365 is a small appliance on your network that accepts plain SMTP from printers, UPSs, servers, NAS and phone systems and delivers it into your own Microsoft 365 tenant with modern authentication. One licence covers every device on the site. Download the free 30-day trial See pricing

Updated 2026-10-10. Menu names and screens vary between models and firmware versions; if yours differs, look for the same fields under the device's email, SMTP or notification settings. Corrections welcome at helpdesk@smtp-365.com.