Yahoo Mail: Object Lesson in Failed Scalability?

Elsewhere I had begun a lengthly blog post detailing my often unsavory history of interactions with customer service at Yahoo! Mail.  A paying customer who intentionally accumulated considerable mail, increasingly that mail could not be searched or retrieved. The symptoms varied, so in the beginning it seemed to be a flaw rather than a design problem.  The example below was typical of the initial symptoms.

After several months of back and forth with level 1 and 2 support, I began to question the initial assessment of "bug."  During this time period, Yahoo developed a new, nicer interface.  There was good functionality in this new work, but my problems persisted.

I'll post more details here later as time permits (I'm in the middle of a tedious migration to escape all this), but it's become clear that Yahoo does not intend for its users to keep large quantities of mail online. Or perhaps its testing environment is insufficient.

Gmail?  Gmail is so far scaling nicely, though I have only a fifth as much mail there.