Add support for message/delivery-status inline

[ietf-svn-bot]

owner:[email protected] type_task | by [email protected]

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Message parts with this MIME type are currently not supported, and so not displayed

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Issue migrated from trac:2589 at 2022-03-03 10:50:51 +0000

[ietf-svn-bot]

@[email protected] changed priority from major to medium

[ietf-svn-bot]

@[email protected] changed status from new to accepted