Cannot send eml attachments on some devices


I work for a hosting company(cPanel interface) and one of our clients has a very weird problem which we cannot figure out.
Most of our clients don’t use EM client, in fact this is the only client we know off that uses EM client on multiple devices within the same netword.

Problem presents the following way.
Since 2 weeks ago problems started on some of the clients devices.
From some stations users are unable to send emails that have eml attachements while other stations have no problems at all.
The weird part is that it is our own mail server that refuses to deliver this messages but we cannot figure out why and it’s not like we are blocking them directly since other users within the same network and domain name dont have this problem.

Emails are sent beetween collegues.

Initially we tought there is something about the EM client that sends the messages, in some way that on some stations they are delivered and on others they are not but we couldn’t find any distinctions between them.

SSL options are active on all devices. Client uses latest version.

Now I know the information I provided is not much and I cannot offer any logs because of GDPR but maybe some of you have faced similar problems and can offer some advice.

Thanks in advance.

P.S. I read some of the pages around and haven’t found anything specific related to my clients issues.

From some stations users are unable to send emails that have eml attachments while other stations within the same network have no problems at all.

If some of the computers within the same network are unable to send email vith .eml attachments yet others can using “the same eM Client version”, “within the same networt”, “using the same SMTP server”, there are three possibilities i can think of.

1). Either computers that cannot send .eml attachments, have differant SMTP server settings setup in eM Client. So check that they all have the same SMTP server settings within eM Client.


2). The computers that cannot send might have “some other software installed” that is interferring with eM Client sending when it sends .eml attachments. Could have some eg: optional eg: Antivirus or Security software etc installed that intercepts or blocks .eml attachments going to the SMTP server.


3). The SMTP server may have some sort of eg: block on the LAN computers that cannot send .eml attachments. Check their LAN IP Address’s are not blocked for certain attachments on the SMTP server. Also are they all DHCP addressing or do some have Static IP address’s. Maybe try changing that to see if it makes a diffeance. I would even assign one of the PC’s which cannot send the same IP address as one of the PC’s that can send to see if it then works to rule out if its an IP Address problem.

Thank you for your answer.

  1. First I was thinking the same thing so I check some of the devices, they are set up the same way regarding the SMTP settings.

  2. Devices within the network use ESET and another anti-virus solution. I disabled the AV on a problematic device but the problem persisted: Mail delivery failed: returning message to sender

  3. I will look more into that but if the local network would be in anyway blocked by the firewall the user couldn’t sent any messages at all. The IP address might be DHCP, I didn’t checked that.