Did you reboot your router after setting those MTU values ? You should always reboot, and not necessary, but good thing is to restart your PC. If that doesn’t help, I really have no clue what could cause your problems… have you tried to contact your ISPs technical support or some of their technicians ?

Also, you may try to completely disable (shut down) your router, then try your e-mail send/receive behaviour without your router running, and see what happens… of course, if you haven’t tried that before.

Do you have any software firewall with e-mail plug-in filter enabled ? If answer is yes, try to disable it temporarly.