One of the most common problems with attachments in Outlook 365 is that it might block file types that it thinks are unsafe or the file size is too large. If you are facing such type of problem then consult Datarecovo experts. They are very experts in solving this type of hindrances.
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You can work your way around this by zipping the file and then attaching it. Here’s how.
Head to where the file you want to attach is located
Right-click on the file and choose to Send to compressed (zipped) folder
Head back into Outlook and attach and send the new zipped folder instead
Some file types that are blocked by Outlook include .ade, .adp, .app. .bat, .com, .jar, .inf, and .ops. The full list is available here. If you’re unable to zip, or if the file type is still too big, you can always upload the file to the cloud, and then share the link instead.
Hope this helps.
Regards,
Bruce