The link can't be sent because at least one recipient isnt valid

Deleted Based on the details you've provided, it's clear that this isn't a simple misconfiguration or user-specific problem, but rather something more systemic affecting the way your SharePoint environment is handling email address validation and external sharing.

While the suggestion to try adding a semicolon after the email address is a valid workaround that has helped others in similar situations, I believe we need to dig deeper to uncover the root cause and implement a more comprehensive solution.

One avenue I would explore is a thorough review of your tenant-level sharing settings, as well as any custom configurations or policies that could be interfering with the normal operation of external sharing. It's possible that there's an underlying issue with the way your environment is set up to handle these types of sharing scenarios, and addressing that at a foundational level may be the key to resolving this persistent problem.

Additionally, I would recommend closely analyzing any network traffic or server-side logs that could provide clues about what's happening behind the scenes when users attempt to share content. A Fiddler trace, as mentioned by one of the previous responses, could be invaluable in this process, as it would allow us to inspect the underlying communication and potentially identify any blocking or validation issues.

Ultimately, I believe that the resolution to this challenge will require a collaborative effort, possibly involving direct engagement with Microsoft Support. Given the tenant-wide nature of the problem, the Support team may have insights or access to diagnostic tools that could shed light on the root cause and guide us towards a lasting solution.