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Doesn't a bounce usually include the Message-ID of the original message, along with a lot of other cruft? I can understand small teams doing this as a quick fix, but companies like Twitter should have no trouble keeping a large dict of Message-ID => account ID.

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'Usually' but definitely not always. Account ID is also usually at the application level, while Message-ID originates lower, at the mailserver level. When the app posts something off to the mailserver, how is it supposed to get the Message-ID back?

At least with the cruft, bounces are just handed back to the app to deal with.

An app can generate its own Message-IDs. MTAs will respect them.

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