Indeed. Changing the MTU solved the issue. Still wonderning why the router does not support the standard MTU 1500. Anyway thanks a lot for the suggestion !

@Kevin_Z Hello Kevin, Thanks for your suggestions. However, I already set the DNS to 8.8.8.8 and 8.8.4.4 on the linux box itself. Nothing changed. I also sent emails without attachments which are...

I am facing very strange issue with my router. When I send emails (gmail) with attached files, I get a timeout on my Linux Ubuntu computer. If no attachment, the email goes out without issue. I tried...