электронные письма не организованы по теме, отправленной из нашего приложения

Наш сайт рассылает кучу разных писем, в основном через PHP. Раньше они были сгруппированы / пронизаны в почтовых программах, и теперь это не так, как я понимаю, ничего не изменилось. Ниже приведены два электронных письма, которые должны были быть сгруппированы, потому что они содержат один и тот же In-Reply-To, но они не находятся внутри Outlook или на iPhone. Обе электронные письма показывают

In-Reply-To: <[email protected]>

который всегда группировал их раньше. Кто-нибудь может понять, почему эти письма не группируются?

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