I see a similar problem too using red-bean.com's dovecot instance after our recent upgrade to the dovecot RC. However, my Sent mail folder is practically empty -- so I'm not sure that the size of the file is of interest. I, too, am using SSL, but am using a much older version of Thunderbird: 1.0.7-1.1.fc3 (20050929). As a final possible deviation from the report here, my mails *do* actually wind up in the Sent folder. It's just that Thunderbird's progress window sits at 100% complete until it eventually times out.
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