Outlook 365 for Mac - not sending attachments

Our organisation has been experiencing this issue for the last calendar month now, one by one a new colleague will experience this issue and the only way we've worked out how to fix it is to revert them to legacy Outlook (we're a fully Mac tech-house). We use HTML email signatures and it looks extremely unprofessional when another staff member falls victim to this bug after emailing numerous external clients and only realising a day or two later that their signatures have broken or attachments haven't sent. How can Microsoft have allowed this bug to escape into the wild and not fix it after over 1 calendar month is completely beyond me, especially when they're charging a monthly subscription for their Office 365 package, I expect better. Emails have had HTML signatures and attachments for over 20 years, how can they have managed to break such simple functionality on such a widespread scale and still not have fixed it?

I have tried to revert Outlook to Legacy and it worked.

I used this information to get Outlook working. I found 2 cases where I needed to set permissions. First - Any folders I attached files from need a Read & Write permission for user Group "Everyone" enabled. Second - For emails received and forwarded, the attachments and embedded images that drop out are stored in this location for the current 2023 version of Outlook - /Users/USERNAME/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/Messages. I just set the permission for Everyone to Read & Write from the Outlook folder down using the Get Info command and then selecting "Apply to enclosed items" to update all sub folders. As soon as this was completed my email started sending attachments and images again for forwarded and newly created emails. It would be good if Microsoft can update Outlook to auto manage this rights piece on MacOS.

I created the following meme to express the frustration of many here:

All humor aside, there are two very acceptable work-arounds posted on this forum:

1. Roll back to the Legacy version (Help > Revert to Legacy). This is preferred.
2. Roll back to version 16.62 or earlier (there are instructions and links posted to download this version, then just drag Outlook to the trash and install it. Turn off updates and presto.

I've done this for the handful of people that work at the company I work for with 100% success.

Update: option 2 does not always work, perhaps need an older version, note the original post mentions the issue with 16.62.

Now it will be interesting to see how long it takes Microsoft to fix this f-real!

Interesting discovery you made here. I too use S/MIME to sign my emails. I postponed using the New Outlook a long time since this feature was not supported for a long time. I haven't tried sending attachments without S/MIME signing yet but it sounds typically like an issue that makes sense with attachments and "new" products released by MS. Sending email without S/MIME is just as useless for me as not getting attachments sent. But yeah, thanks for sharing this discovery.