Getting Error Code 550 5.7.350, how do I resolve this

Question

I have seen several threads about this and have yet to find a solution. I am getting this error:

550 5.7.350 Remote server returned message detected as spam -> 550 5.7.1 [2a01:111:f400:7d05::704 19] Our system has detected that this;message is likely suspicious due to the very low reputation of the;sending domain. To best protect our users from spam, the message has;been blocked. Please visit;https://support.google.com/mail/answer/188131for more information. a23-20020aa7d917000000b00516a04ea63csi3068992edr.637 - gsmtp

Message rejected by:

mx.google.com

I have checked my SPF, DKIM, and DMARC and all seem good. I have also checked my Domain reputation on Barracuda and it says my reputation is good as well. Google has been less than helpful ( I have had tickets in for weeks with no response as of yet ). My issue is not with a specific google domain but with literally all google domains I have encountered ( as well as my personal gmail not allowing it as well )

Answer 1

We do not have a hybrid configuration, running cloud entirely. But we added a connector to force outgoing traffic to google based mail hosts to use IPv4 apparently fixed the issue. Seems indeed dns resolution of ipv6 addresses originating from Microsoft Exchange Online public mail services is not configured for some tenants? see this thread also Emails being blocked by Gmail from office 365 - Microsoft Community

Answer 2

I did manage to solve the problem, it turns out my department is using only ipv4 and when it relays through Microsoft, it gets sent as IPV6 which was causing the errors. I reached out to microsoft to send my departments emails through ipv4 and it seems to have fixed the issue. ( also , not sure if you have but you need to update your mx, spf , and all other dns records as well.)

Answer 3

First, the admin of the recipients adding our domain to the allow list will only go so far as it is mx.google.com ( and I have done this with 1 of the admins and it still never reached him )

• The NDR error you mentioned I will look at ( even though it doesn't match the error I am getting )
• The desktop Client is fully updated, but the problem is from desktop, phone, OWA, etc. Its from all ways I have used thus far ( windows and Mac as well)
• The error arises ANYTIME anyone in my organization emails anyone from any google account ( the error is saying its mx.google.com that is rejecting )
• The issue is isolated to sending, we can receive from google with no issue

Answer 4

Hi Kyle Cole 1,

Were you able to resolve this issue. We are experience it currently and except a request to whitelist our domain, it seems nothing else can be done?

Answer 5

Dear Kyle Cole1

Good day! Thank you for posting to Microsoft Community. We are happy to help you!

Regarding this error, to better help you, I still need to collect some details to narrow down the scope of question

• First please kindly let the admin of the recipients' to add your domain in their safe sender listand allow list in the spam filter to see if it helps.
• In addition please also kindly check this official document Fix NDR error 550 5.0.350 in Exchange Online - Exchange | Microsoft Learn
• Please also check whether your Outlook Desktop Client has been updated to the latest version, and if not, please check the results after updating.
• In the meantime, I would like to ask you if your question only occurs when sending emails to this particular user, or do you get this error when emailing everyone?
• Are you just unable to send or are you having trouble receiving mail?
• If it is convenient, please kindly share with me a screenshot containing the error code of the error message you received, thank you.

Please trust me to do more research and testing when I receive feedback giving me the information I need, thank you.

Thanks in advance for your understanding! Your patience and cooperation will be highly appreciated. Hope you all the best!

Sincerely,

Kerry Chen | Microsoft Community Moderator