MS Outlook Error 0x800ccc0f (The Connection to the Server was Interrupted) when sending email with attachment

Summary:

Some of our customers using our SMTP server have been facing this error while try to send emails that have attachments.
Emails without any attachment would send out fine without any problem, however whenever a user would try to send an email with an attachment, it would stay in the outbox trying and would eventually pop this error:

"Error 0x800ccc0f (The Connection to the Server was Interrupted)"

Problem or Goal:

The confusing part is that it works fine on the office LAN but fails on the 4G network.
works fine on other SMPT servers on the 4G network but only one specific SMTP was failing to upload files.

Cause:

Upload is more senstive to network statability than download, an upload is more likely to fail in a network with losses that a download.

Solution:

I ran two seperate traceroutes off the office LAN and 4G networks and indeed the traffic was taking a different PATH towards the last 3 hops to the SMTP server.

I also ran a test PING to the SMTP server on the different networks 4G and office LAN.
Office LAN was without losses yet 4G had losses towards the SMTP server.
I tested another PING to another SMTP server on 4G and it was lossless which points me to an issue on the PATH to my SMTP server.

The ping would reply for about 1 minute and then timeout for another minute, this makes the upload to fail because for the upload to work, there has to be connectivity for the whole time the file is trying to be uploaded.

I also ran a tracert to the SMTP server during the ping timeout and identifies a problematic hop in the last 3 hops towards the SMTP.

Next step is to engage my SMTP server hosts to sort out this network issue with their ISP.

Problem Solved?

Am still waiting for the feedback but hopefully it gets resolved.