Outlook Mail sending problem with attachment

Hello,

I am having a serious problem in sending attachments (>3mb) from outlook. I use yahoo as my email client. 

Outlook Version

Error: "An operation on the server timed out. The server may be down, overloaded, or there may be too much net traffic"

The problem mainly occurring in my office network.

To check if the office network has a problem I have tried sending the same email using Thunderbird and Mac mail and both of them succeeded in sending the email(though very slow).

It was okay to send an attachment 10 days earlier. But I am having this problem recently. I have tried changing port details but nothing helps.

I need help badly.