Gmail rejecting Microsoft 365 emails with 550 5.7.350 — low IP/domain reputation?

Posted by TechOfficeInc@reddit | sysadmin | View on Reddit | 14 comments

We're using Microsoft 365 Exchange Online to send from techoffice.ca. Gmail is rejecting all our emails with:

550 5.7.350 Remote server returned message detected as spam -> 550 5.7.1 [2a01:111:f403:241d::718] Gmail has detected that this message is likely suspicious due to low reputation.

✅ SPF, DKIM, DMARC all pass.
❌ Google Postmaster Tools shows no data (mail rejected at SMTP level).
📌 Sending IP is an IPv6 from Microsoft’s shared pool — looks like a bad rep issue.

We can’t force IPv4 or control IP rotation from our side, and Microsoft support hasn’t been helpful yet.

Looking for:

Anyone else hit this with Microsoft 365?
Can MS route Gmail over IPv4 or clean IPs?
Tips for escalating this properly?
Should we just use a smart host for Gmail temporarily?

Would love to hear how others resolved this.

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Frothyleet@reddit

Are you sending marketing or other mass email communications? If so, don't.

If you are, MS is likely using its "low confidence" IP pool for your outbound mail, where it puts tenants that aren't outright violators but whose email practices are annoying enough that they could poison the rep of the tenants in the high reputation pool.

Use a third party service like mailgun for your marketing communications, and also use a subdomain with separate SPF/DKIM/DMARC to keep the rep of your root TLD clean.

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TechOfficeInc@reddit (OP)

Yes, we are sending our service proposals to multiple contacts. Currently, we are unable to reply to any incoming Gmails. Going further we can definitely think about using the different service method when sending out marketting emails. What can be done, as I am unable to find much more detail?

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lart2150@reddit

did you setup postmaster tools months ago so you have info about you domain rep? https://gmail.com/postmaster/

setting it up now will help you in like a month or two once it has info 🙄

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Happy_Kale888@reddit

Thank you kind sir!

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Frothyleet@reddit

Well, step 1: stop sending marketing stuff from your tenant. And whatever tool you use to replace it, don't use your primary domain (use a subdomain, like marketing.yourcompany.com).

After that, you are just stuck waiting for your reputation to improve. Alternatively, you can pivot to a new domain, but new domains are usually low-rep to avoid spammers just jumping around on domains.

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serverhorror@reddit

You are spamming, and because of that, you are in the low reputation IP pool.

Doesn't even matter whether or not these are "valid" subscribers. It's likely that enough people just hit the spam button, for one reason or another. That's how you end up in these pools.

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siedenburg2@reddit

If you want to send marketing "spam" you should use a dedicated service or at least microsoft high volume mail for that. It doesn't guarantee that everyone get the marketing mail, but at least you normal ones will be delivered.

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AviationLogic@reddit

Not sure if this is the actual cause, but something to look at.

It Depends on the IP Pool. Microsoft will route email it thinks from "Risky" ranges and receivers will likely reject emails from that IP range. Mass emailing can cause this pretty easily.

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TechOfficeInc@reddit (OP)

Can you please advise me on the further steps to be performed.

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