Cannot Send Attachments

We are experiencing a very strange problem. This is NOT related to authenticating SMTP, or any antivirus or firewall software. Let me know that it has been tested in several machines and we got our conclusions, and the same did some other technicians.

We own a dedicated server with around 70 domains. We do not resell hosting. We just develop sites and host them there, usually with their emails too.

This first all started with a customer that could not send attachments on his email. He told us that could not send email with attachments. Normal emails worked, but not this.

Of course we helped them configuring the account once and again. In some cases it seemed to work, but then it didn't. We thought it could be related to their ISP, because it happened the same in different shops they own in different locations. This happened to them when sending files (reports) each other (all in the same domain).

The error they were getting was about "Socket Error 10053. 0x800ccc0f". I was able to reproduce this error unchecking the "smtp needs authentication", and also "use the following account", while inserting a username but not password. Anyway, as I stated before, all this has been checked and re-checked. So seems to have no relation with antivirus or authentication error.

In other cases, people has been able to send them the email, and they receive nothing.

Then, after a few days, the sender receives some kind of mail delivery error.

Webmail also does not seem to work for them. They see the page strangely loaded, they say, and then attacing a file and hitting Upload, they get a "page not found", when I can see it perfectly and run it, on my office.

Then another customer called... webmail was also weird, and his own web site was doing strange things. Some images were not loading, but when refreshing, you could see those images loaded, and maybe others not. Something random every refresh. At same time, this customer had problems when sending an email with attachment to another customer that is in our own server.

Today some more customers called... one was very angry because a customer (his customer, not ours) could not send email to him. So mails yes, others not. Guess?. Yeah: attachments.

The other call was about the same... a designer that could not receive attachments (to work), because the other side was getting errors when attaching.

And another customer called because one of their customer could not send attachments.

This customer's customer is a big company (with around 1,000 employes), and their technicians checked their own accounts. They said everything was OK so it had to be something wrong in our customers account.

Of course we work normally, we have no problems, and no other calls received.... web sites are OK, server seems to be OK... I phoned the ISP that maybe was having trouble, I also phoned my hosting provider... but you know "we will check", "yes we are checking it", etc.. for about 4-5 days now.

And I'm really lost.

Does anyone have an idea of what could be happening? Is there any way to "block" attachments to a router/server/connection? This is the only idea that can go on my mind... in that case, how do I check this?

This started happening around 12-15 days ago. At same timeframe, there were a couple of servers hacked in this hosting provider, but I'm not sure if this has something to do with it... today we received an email about "network manteinance" at 7pm.

Any help or clue will be appreciated, as I'm only a web developer, with basic tech knowledge, so not a veteran system admin...