550 5.0.350: generic error from the recipient's server

Microsoft calls 5.0.350 a generic wrapper that Exchange Online uses for a wide variety of non-specific errors, typically returned by the recipient's email organization. The NDR's "Diagnostic information for administrators" section contains the original error from the remote server. That text, not the 5.0.350 code, tells you what really happened.

Two variants have their own fixes. If the NDR also says "x-dg-ref header is too long", the message was sent in Rich Text Format from Outlook, likely with an attached email message that itself has an attachment. Microsoft explains that in Rich Text messages the attachment's data becomes part of the X-MS-TNEF-Correlator header, which can grow too long for the receiving server. The fix is to send in HTML format; admins can also control TNEF settings for remote domains and mail contacts.

If the NDR says "Requested action not taken: policy violation detected (AS345)", the message has an attachment, such as a Word file, with 20 or more embedded files. Remove some embedded files and resend. For every other 5.0.350, Microsoft says there's typically nothing support can do, because the problem lies with the recipient's email system.