Revert "Merge pull request #23218 from karlfreeman/bump_mail"
Revert "Merge pull request #23218 from karlfreeman/bump_mail"

This reverts commit 62aa850fee2070ec3e2d9e4f925dfd7790d27b5e, reversing
changes made to 8c1f248c58ba65a786ae295def325c8982e7f431.

There is no reason to disallow mail 2.5 so we don't need to bump the
version constraint since people are still able to use mail 2.6 and get
all the memory saving that was pointed in the pull request description.

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