Emails sent from Horde are getting blocked but emails sent from email client work fine

[Outlier]

Emails sent from my email client has been generally reliable. However, when sending emails from Horde (accessed by www.domain.com/webmail) some recipients don't receive the emails (particularly some @outlook.com email addresses). This is the error message I got from outlook.com:

"Unfortunately, messages from [208.***.**.28] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140)."

If emails sent from my email client are sending to those same email addresses with no problem, then most likely my email client is using a different IP than Horde is using. When I do a SPF record check on my domain, I see about 20 different IP addresses including the one listed above in the error message. First of all, is it normal to see so many IP addresses associated with your domain in a SPF check? If so, is there a way to configure Horde to send using a different IP, like the same IP as my email client or any other IP that has not been blocked?

 

[cPRex]

Hey there! Does this also happen with Roundcube webmail? We're removing Horde in version 108, so this problem could end up resolving itself for you in the next few weeks.

 

[Outlier]

I just checked and it's the exact same situation with Roundcube webmail. The problem seems to be with outlook.com blocking my outbound webmail IP address. However, it is not blocking emails sent with my email client. So I'm wondering, is there any way to control which outbound IP is being used when sending emails? When I do a SPF check of my domain, I see about 20 IP addresses associated with it. Is this normal?

 

[xpy-xpy]

Based on the behavior you are describing, it is possible that your web host offer customers 3rd party email service for better deliverability, but has not bothered to reconfigure webmail in CPanel correctly to use such a service.

To test the hypothesis I would perform a reverse DNS check on the IP in the error message you got from outlook.com . There are a lot of free online tools to quickly check that. Now, if the reverse DNS result looks generic such as "xxx-xxx-xxx-xxx.static-customer.yourprovider.com" then it was most certainly never setup to be used as an email server. There is zero chance to deliver anything to Microsoft's email services if your IP has a generic rDNS.

Having many IPs on an SPF record is fine and further supports the idea that your host is probably using a 3rd party email service. If you check rDNS of some of these IP, do they look different from the IP that was returned in the error message?

 

[cPRex]

If you check your server's IP address through a tool like Email Blacklist Check - IP Blacklist Check - See if your server is blacklisted do you see that you are on any major blacklists?