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Dear MichalRama,
Good day, thanks for querying in this forum. we're happy to help you.
From your description, it seems that you encountered the issue when sharing the files in SharePoint, sometimes, it doesn't send any email.
If my understanding is right, we have also tested it on our side and we cannot reproduce the same situation. Given this situation, we do understand the inconvenience caused by it. Since you mentioned this issue occurs in different devices and accounts, this issue may be related to your tenant. The best way to troubleshoot the issue is to report the issue to the related Team. We’d suggest you contact your office 365 administrator to go to Office 365 Admin Center-> Support-> New Service Request, the support engineers there have the correct escalation channel and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket. Ways to contact support for business products - Admin help. The engineers in related team have higher permission and they may help check the root cause from background.
If the scenario above is not consistent with yours, you can also post back and point that.
Thanks for your cooperation.
Sincerely,
Sukie | Microsoft Community Moderator