Outlook for Mac -Smtp submission failed. Server 'smtp.gmail.com' Port '587'. --> Unexpected SMTP server response. Expected: 235 = Failure code: 64d3

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Outlook for Mac -Smtp submission failed. Server 'smtp.gmail.com' Port '587'. --> Unexpected SMTP server response. Expected: 235 = Failure code: 64d3

John Carroll 0

Hi - I'm using Outlook for Mac. I have a gmail account. It's been working fine until yesterday. It now has problems syncing with the Google server, and sent emails are bounced back with the following message:

SmtpSubmissionPermanentException: Smtp submission failed. Server 'smtp.gmail.com' Port '587'. --> Unexpected SMTP server response. Expected: 235, actual: 334, whole response: 334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==Failure code: 64d3

Gmail works fine with a login through any browser.

I have reset the account, deleted the account and added it back, switched from new to legacy view, and researched on the internet and found no answer.

Many, if not most, of the advice online, says it for Outlook for Mac, but then gives instructions for the PC version of the program. The menu setups are different with both programs and they don't have the same features. It also appears with the new Outlook for Mac, I cannot access the SMTP settings directly.

Any advice would be appreciated. Thank you.

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Answer 1

Hi John Carroll,

Welcome to Microsoft Q&A Forum!

Have a good day and I hope you're doing well!

Thank you very much for reaching out and providing detailed information about the difficulties you are experiencing with Outlook for Mac. I completely understand how disruptive and frustrating it can be when email services are not functioning as expected, especially when everything was working smoothly before. Please rest assured that I am here to support you and will do my utmost to help resolve this issue.

Based on the error details you provided (Unexpected SMTP response with code 334 and failure 64d3), this appears to be related to authentication issues between Outlook and Gmail, often tied to OAuth tokens or recent Google security updates. Here is the most likely solution to try first:

Reset OAuth Permissions: Since Outlook for Mac uses modern OAuth authentication with Gmail, the token may have become invalid or revoked.

Access your Google Account settings at myaccount.google.com, type 'manage your Third-party connections'.
Find and remove access for "Outlook" or "Microsoft Outlook".
Then, in Outlook for Mac, remove your Gmail account and add it back again. This should prompt a fresh OAuth login and grant permissions. Make sure to allow all requested scopes (e.g., access to mail) during the process.

To assist you more effectively, may I kindly ask for a few additional details?

1. Which version of Outlook for Mac are you currently using? You can find instructions on how to check your Outlook version here: Check your Outlook version

2. Which version of macOS is installed on your device? (To check, click the Apple logo in the top left corner of your screen and select “About This Mac.” The version information will be displayed.)

3. Have you updated Outlook to the latest available version? If you are already using the most recent version and the problem persists, it may be helpful to try reinstalling an older version to see if that resolves the issue.

4. Additionally, have you enabled 2-Step Verification (2FA) on your Gmail account? (This can be checked at myaccount.google.com/security.)

Before making any changes, I recommend checking your data on Outlook Web to ensure that all your emails, calendar events, and contacts are fully synchronized, to avoid any potential data loss. If needed, consider backing up important items manually.

There are also other possible reasons for this issue, such as an outdated Outlook version, temporary service interruptions from Google or Microsoft, or account configuration conflicts. However, based on my research and similar cases, resetting OAuth often resolves it quickly, especially if 2FA is enabled.

I understand that troubleshooting these issues can be time-consuming and sometimes overwhelming. Please know that I am here to support you throughout this process, and I am committed to guiding you toward the best possible resolution. Once you provide the requested details, I can offer more tailored advice. If you have any further questions or need additional assistance, please do not hesitate to reply to this email. I am always here to help.

Wishing you a smooth and productive day ahead.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Nghia-P 2,520 Reputation points Microsoft External Staff Moderator

2025-08-08T08:27:45.73+00:00

Hi John Carroll,

Would you mind getting back to us when it’s convenient for you?

Your feedback is important for us to continue troubleshooting effectively. If you need further assistance, please share more details so we can provide the most accurate support possible.

If you feel this helped point you in the right direction, you're more than welcome to mark it as "Accepted Answer". This helps me keep track of your case and continue supporting you as best I can. This can make it easier for others facing a similar issue to find their way to a solution, too.
John Carroll 0 Reputation points

2025-08-08T20:26:59.2733333+00:00

Hi - I was able to get Outlook to work (hopefully permanently) by upgrading the macOS and reinstalling the account and switching it back from Legacy to the new look.
The issue for me in following your suggestions, is that this is not an account for one person. We have a Google Workspace account, where security changes are global and will affect every Outlook user in the company. It's just not something I can play around with as it would have major consequences.
I do appreciate your suggestions though. Who knows, this problem may come back and then I will definitely need your help. Thanks again.
Nghia-P 2,520 Reputation points Microsoft External Staff Moderator

2025-08-11T00:41:46.1166667+00:00

Hi John Carroll,

Thank you for the thoughtful update. I know issues like this can be really disruptive, so I’m genuinely relieved to hear Outlook is back up and running after you upgraded macOS, re‑added the account, and switched to the New Outlook. Those steps essentially refreshed the Google OAuth token, which aligns with the underlying issue we suspected for the 334/64d3 error.

I also understand your concern about org‑wide security settings.

Just to clarify: removing “Microsoft Outlook” under Google Account > Security > Third‑party apps is a per‑user action and won’t impact other users in your Workspace. Only changes in the Admin console apply globally. If the issue returns, it’s safe to try that per‑user token reset on your account.

If you feel this helped point you in the right direction, you're more than welcome to mark it as "Accepted Answer" so others facing similar issues can find it more easily. This also helps me keep track of your case and continue supporting you as best I can.

If you still have any concerns or questions, we’re here and happy to assist. Please feel free to reach out anytime.

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Outlook for Mac -Smtp submission failed. Server 'smtp.gmail.com' Port '587'. --> Unexpected SMTP server response. Expected: 235 = Failure code: 64d3

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