I have to disagree on the email provider/client being the problem. In my case, I tried three different providers, all of them architechturally different (one a MS Exchange acct, one a mass market webmail acct, and one high performance "email only company" acct). And in addition to Outlook, each of these three accts have wildly different webmail interfaces and underlying construction.
I initially thought the same as you when I first encountered the problem with my main acct (the Exchange one), but after I tested with the other two accts, there's absolutely no similarity between the three accts (and nothing exotic or unique to distinguish them from those of you who aren't having problems).
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