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.

Sendmail SMTP Port 25 - Unable to connect to remote host: Connection refused

Summary: 

When you telnet a local address on port 25 (SMTP) you get error below:

"Unable to connect to remote host: Connection refused"

Problem or Goal:

When you attempt to connect to port 25 on a local address, you get error:
 "Unable to connect to remote host: Connection refused"

root@mail:/etc/mail# telnet 10.0.40.104 25

Trying 10.0.40.104...

telnet: Unable to connect to remote host: Connection refused

Yet you can successfully connect to port 25 on localhost or 127.0.0.1

root@mail:/etc/mail# telnet 127.0.0.1 25
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.

Cause: 

By default, sendmail will only listen on localhost.

Solution: 

Set it to listen on all interfaces instead:

1. Open /etc/mail/sendmail.cf
2. Find the line starting with DaemonPortOptions and then modify it so it looks like this:

Before:
# SMTP daemon options
O DaemonPortOptions=Family=inet,  Name=MTA-v4, Port=smtp, Addr=127.0.0.1

After:
# SMTP daemon options
O DaemonPortOptions=Family=inet,  Name=MTA-v4, Port=smtp, Addr=0.0.0.0

3. Make sure to restart sendmail so that our new configuration is taken in to effect:

/etc/init.d/sendmail restart

Problem Solved?

YES

root@mail:/etc/mail# telnet 10.0.40.104 25
Trying 10.0.40.104...
Connected to 10.0.40.104.
Escape character is '^]'.