550 5.0.350 and 5.7.350 errors from Microsoft
Microsoft's 5.0.350 wraps an error the recipient's server returned, so it isn't the real reason. Find the original code inside the bounce and handle that.
What 550 5.0.350 means
550 5.0.350 Generic errorMicrosoft's wrapper for non-specific errors returned by the recipient's organization. The real reason is in the bounce's diagnostic section.
550 5.0.350 x-dg-ref header is too longSent from Outlook in Rich Text Format with emails attached inside. Send as HTML instead.
550 5.0.350 Requested action not taken: policy violation detected (AS345)An attachment with 20 or more embedded files. Trim it.
550 5.7.350 Remote server returned message detected as spam -> 550 5.7.1 …5.7.350 isn't in Microsoft's documented list. It's been seen wrapping another server's spam verdict, such as Gmail's low-reputation 5.7.1. Handle it as the code after the arrow.
List problem or sender problem?
Depends on the text. The wrapper says nothing on its own. The original error inside it decides, and Microsoft says that's usually on the recipient's side. It's a permanent failure: the server won't accept this message if you send it again unchanged.
How to fix 550 5.0.350
- Open the bounce's "Diagnostic information for administrators" and find the original code from the recipient's server.
- Look that code up and handle it as that code: the wrapper itself isn't the cause.
- For the Rich Text and AS345 versions, send as HTML or trim the attachment.
In SendingOS, 550 5.0.350 shows on the conversation with its reason and code, so a problem on their side never gets mistaken for one on yours.
Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.
Related codes
Sources: Microsoft: fix 550 5.0.350 · Microsoft: NDRs in Exchange Online · Microsoft Q&A thread on 5.7.350