Outlook Express links you directly to the mail server of your ISP. Is this server using MTU set to 1454, and thus have a problem in establishing a proper TCP handshake with your comp since you have turned off PMTUDiscovery?
That is the prime purpose of PMTUDiscovery. Not all comps and ISPs use one single MTU value. With the PMTUDiscovery turned off, there will not be a proper TCP handshake and agreed TCP Protocols. The extra overhead is negiligble compared to connections stalling.
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