Problem sending emails with attachments

Hi Guys,

Got a real tricky one - I spent hours yesterday trying to figure out why we cannot send emails from our pcs to our email server if they contain attachments. Turns out, it’s a problem with our Mikrotik RB2011 - if I change back to our old router everything works fine. Change back to the RB2011 and emails stop sending again.

The error we get is this:

reported error (0x8004210B) : 'The operation timed out waiting for a response from the sending (SMTP) server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'

Seems as though there is a connectivity issue but I don’t know where to start.

Can somebody please help me debug this?

Cheers,
Andy