What Does 550 5.7.350 Mean, and How Do I Fix It?

What the bounce says

The full text is 550 5.7.350 Remote server returned message detected as spam. Exchange Online writes it, so the message went out through Microsoft 365. "Remote server" is the server that was supposed to receive it: the recipient's mail system or the filter in front of it.

The wording says a server outside your tenant judged the message as spam, and Exchange Online passed its answer back to you in the bounce.

What Microsoft documents, and what it doesn't

Microsoft has no troubleshooting page for 5.7.350, and the code is not in its NDR reference for Exchange Online: the 5.7.3xx rows on that page go from 5.7.321 to 5.7.325, then 5.7.367. Google's Gmail SMTP error list doesn't include it either. Any list of "causes of 5.7.350" you find is someone's reading of their own cases.

Two documented codes help read it. Microsoft calls 550 5.0.350 "a generic wrapper" for "a wide variety of non-specific errors that are typically returned by the recipient's email organization." It adds that the "Diagnostic information for administrators" section of the bounce "will contain the original error message" from the external server. The sibling 5.7.367, "Remote server returned not permitted to relay", is listed as an SPF or DKIM failure on forwarded or relayed mail.

Where the real reason sits

The useful part is the reply that follows the code. In a Microsoft Q&A thread from May 2024, the bounce read:

550 5.7.350 Remote server returned message detected as spam -> 554 5.7.1 Message refused by SURBL check

Everything after the arrow is the recipient's server speaking. Here it checked the links in the message against SURBL, which "lists domains of malicious or abused web sites." Another server may name your IP, a content score or failed authentication.

If the bounce shows no reply after the code, open "Diagnostic information for administrators" and the remote server name. For generic 5.0.350 errors Microsoft says "the problem lies with the recipient's email system", and the same reading fits 5.7.350.

Checks you can run

1. Copy the whole bounce, including the diagnostic section, before you change anything.

2. Find out who refused it. Look up the recipient domain's MX with MX lookup. The MX host shows which filter answered, which tells you whose rules you hit.

3. Read the original reply and match it to the table above.

4. Send a neutral test. Send a short plain-text email without links from the same mailbox to an address you control behind a similar filter. If it goes through and the campaign email doesn't, the copy or a link is the cause.

5. Check the domains in your links. A tracking domain shared with other senders can carry their reputation; see open tracking and links.

6. Don't resend the same email. Change what the reply points at first.

What it means for cold email

For a cold campaign 5.7.350 is a spam verdict from the recipient's side. A few in a send point at those recipients' filters. Many across different domains point at something shared by every email: the sending IPs, a link domain or the copy. Microsoft 365 tenants used for cold email also face their own limits; see Outlook blocking cold email and 550 5.7.233.

What we recommend at Outreach2day

Every Outreach2day mailbox gets placement tests in Google and Outlook and a warmup inbox and spam rate in analytics. Compare the placement tests before and after you change the copy or links; they use seed mailboxes, so read them as a trend.