We've chosen Apprise just for the big number of services supported, actually at the moment we only use email and telegram as notification output channels.
Apprise does not accept SMTP protocol as input, so you're bound exclusively to API, binary exec or third party integrations.
I think Posthorn could fill a gap if it will integrate the possibility to send a webhook (alongside/instead of email).
I'm not an heavy o365 user but i'm almost happy on Debian KDE with thunderbird 148[0] (email only), teams-for-linux[1] (chat/calendar/whatever), Onedrive[2] and webdav (sharepoint)[3]. Libreoffice/Onlyoffice for documents.
[3] Store the SP cookie via konqueror visiting the SP site, then open it in dolphin via "webdavs://CORP.sharepoint.com/sites/SITE/Shared Documents/" (sometimes the cookie is very short-lived)
Apprise does not accept SMTP protocol as input, so you're bound exclusively to API, binary exec or third party integrations.
I think Posthorn could fill a gap if it will integrate the possibility to send a webhook (alongside/instead of email).