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ISA550W failure to upload/send via mail large files (bigger than 4MB)

Hi to all and wishes for peaceful 2014!

We are facing a problem with an ISA550W(Firmware 1.2.18). The issue is that when trying to upload large files (bigger than 4MB) by e-mail as attachments or via a file sharing service (for example wetransfer) it times out and fails! I have disabled all security services for troubleshooting purposes but the issue remains.

I have tried to set up manually the MTU of the line to avoid "black hole routers" and with an MTU set to 1485 to the ISA the maximum response to ping is with MTU 1272 from a windows PC. From my understanding it seems that the maximum MTU from the clients can not be larger than 1300, which is very low.

The issue does not appear if the ISA is replaced with another router provided by the ISP. The MTU is set to AUTO which is 1492.

I am really in a tight spot as we are a creative office and are in desperate need for uploading large files.

Any suggestion will be very much appreciated.

Many thanks in advance!

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