Solved 3CX Custom SMTP Server stops working

[asdfasdfasdfasdf]

Wondering if anyone else had their voicemail-to-email suddenly stop working and spend a couple hours scratching their head over why, only to eventually give up and switch to the 3CX SMTP server? This has happened on at least 3 different servers now (15.5).

https://www.3cx.com/docs/manual/ seems to be extremely limited in information.

I go to O365 and create a basic connector to whitelist the static IP assigned to my 3CX server.
Then I go to 3CX -- > Email --> Custom SMTP Server and enter:

Mail Server: mydomain-com.mail.eo.outlook.com

Reply to Address: [email protected] (this email has never existed anywhere in my O365 tenant and worked for years up until Feb 3 2020)

Enter your email or username: BLANK

Password: BLANK

Enable SSL/TLS: NO

So then I read somewhere you should change it to mydomain-com.mail.protection.outlook.com. That fails with "Connection timed out: Ensure that Port 25 TCP is open for outbound traffic."

Windows Firewall is OFF?!?!?! I enable the Windows firewall, set up outbound 25 allow, same error. Then I read about some 2528 port that needs to be allowed outbound. Add that, same error. AWS outbound rules (security group) are set to ALLOW ALL OUTBOUND.

Also tried smtp.office365.com and re-creating the connector in O365 without luck. Rebooted server, no luck.

Does anyone know the proper configuration settings to use 3CX Custom SMTP server with Office 365 connector -- a scenario where no actual O365 user OR license needs to exist for this setup? Again, it's been working for years.

 

[asdfasdfasdfasdf]

ANOTHER CUSTOMER'S 3CX instance has stopped working -- 2nd one this month, using Custom SMTP server and the mydomain-com.mail.protection.outlook.com format. ANYONE ELSE?

 

[cobaltit]

What makes you think this is a 3CX problem? If you aren't authenticating, you are still subject to O365 spam filters and internal RBL.

https://docs.microsoft.com/en-us/ex...e-or-application-to-send-email-using-office-3

Setup an account as option 1 or deal with the random blocks. The port 25 issue has nothing to do with a firewall. Changing the server doesn't change what port you use so that's just a bad error message. And 2528 is what 3CX SMTP uses so that only matters if you are using 3CX SMTP, not custom.

 

[asdfasdfasdfasdf]

Yup, I've seen that article a million times. As promising as it looks, options 2 and 3 do not work. I have everything set up exactly like they say.

 

[cobaltit]

I'm not sure what you mean by you have everything setup the way they say. Option 2 and 3 do work but they are subject to MS spam filtering and RBL as the table at the bottom shows. If you want it to work 100% of the time then you have to do option 1. Option 2 and 3 will work until the day it doesn't, usually because of the RBL. You can check their RBL here:

https://sender.office.com/

 

[asdfasdfasdfasdf]

Just checked, thanks for the link. "The IP address in question is not currently blocked in our system.". What I meant by having everything set up the way they say, is that I followed their instructions exactly: obtain MX record, obtain static IP, add IP4 to SPF record, create connector in O365...

Thanks for your suggestions, I was just hoping to find someone who has run into this same mystery and actually found a reason for it. I have seen all the documentation.

 

[cobaltit]

Without authenticating, you are subject to the whims of MS and their various filter mechanisms. We ran into this a LOT with scanner/multifunction devices which is what that article is geared for. Email would work for days/weeks/months and then just stop. The permanent solution is to authenticate (which back in the day most multi-function devices didn't support the newer SSL/TLS O365 wanted) so we ended up using a 3rd party SMTP service. Long story short it's not a 3CX issue.

 

[asdfasdfasdfasdf]

Thanks @cobaltit , I agree, not 3CX.