Parse disposition-notification-to header as address

[RoyalBingBong]

In our application we need to handle MDNs and for that we need to parse the "Disposition-Notification-To" header and compare the address with the Return-Path header.

I would like to have the same structure of addresses and use mail parser function, but it feels quite dirty to use undocumented and seemingly internal-only methods like decodeAddresses, getAddressesHTML and getAddressesText, so I decided to add a small PR in the hopes that it will be accepted and merged in a timely manner.

Unfortunately I do not have the time (and quite frankly the knowledge) to implement all the parsing related to MDNs (e.g. machine readable and human readable content for an MDN mail).

[CLAassistant]

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