Microsoft Outlook for MAC Updated Client Not able to send Emails with Attachment For Exchange 2019 Mailbox

Question

Microsoft Outlook for MAC Updated Client Not able to send Emails with Attachment For Exchange 2019 Mailbox

Muhammad Sohaib 20

It looks like Outlook (Legacy) and NEW is stuck trying to send an email from your Outbox to [Moderator note: Personally Identifiable Information removed] @contoso.com, and it's timing out with error code -1001 ("operation on the server timed out... server may be down or overloaded, or there may be too much net traffic"). MAC OS : 26.6.2

Outlook Version: 16.112.2 (26082125)

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2 answers

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Answer 1

For Outlook for Mac sending failures with attachments, use this order:

Check whether the issue matches a known Norton Security for Mac problem.
- Error -17099 or -3253 when sending with attachments was caused by the Vulnerability Protection module in Norton Security for Mac version 8.6.3 or newer.
- Norton released a hotfix for that issue.
- If Norton is installed on the Mac, update Norton and apply its hotfix first.
Check whether the message is simply too large to send.
- In Outlook for Mac, oversized messages are held in Drafts.
- Open Drafts, remove some attachments, and send again.
- If deleting the stuck message, also delete it from Trash to stop repeated attachment error behavior.
Verify Outlook is online.
- In Outlook, open the Outlook menu.
- Make sure Work Offline is not checked.
Confirm basic network connectivity.
- Test internet access in a web browser.
- If the Mac is on a restricted network, SMTP traffic may be blocked or delayed.
If the account is POP or IMAP, verify outgoing server authentication.
- Go to Tools > Accounts.
- Select the account.
- Under Outgoing server, select More Options.
- Choose the required authentication type and enter credentials if needed.
If the network or ISP blocks the outgoing SMTP server, use the alternate SMTP server provided by the network administrator or provider.
- Go to Tools > Accounts.
- Select the account.
- In Outgoing server, enter the alternate server name or address.
Verify account credentials and server settings.
- Go to Tools > Accounts.
- Select the account.
- Confirm the email address, user name, password, and any required SSL or port settings are correct.
If the mailbox is Exchange and Outlook for Mac is not syncing correctly, check the Exchange connection and clear cache for the affected folder.
- On the Tools ribbon, choose Accounts.
- If the Exchange account indicator is orange, there is a connection problem.
- To clear cache: in the navigation pane, right-click the Exchange folder, select Properties, then on General choose Empty Cache.
- Warning: emptying the cache deletes any information not synchronized with the Exchange server, including contacts' mail certificates. Outlook then re-downloads items from Exchange.
If the issue occurs only for signed or encrypted messages with attachments or inline images, update Outlook for Mac to build 16.66 or later, where that issue was fixed.

If none of the above resolves it, use Help > Contact Support in Outlook for Mac.

Answer 2

JeseminQH 515 Independent Advisor

Hi @Muhammad Sohaib

May I kindly ask whether you have already tried the troubleshooting steps suggested by the Q&A AI? In particular, please follow the steps provided in the articles below to see whether they help resolve the issue:

Additionally, to help me better understand and narrow down the issue, may I kindly ask:

Are other users in your organization experiencing the same behavior?
Have you tested Outlook on another device? If so, were you able to send the email successfully?
Can you send emails successfully through Outlook on the web?
Is the problem affecting all outgoing emails or only emails sent to this specific recipient?
Have you tried using a different network, such as a mobile hotspot?
Does the stuck email contain any large attachments? If so, what is the approximate attachment size?

Please try the troubleshooting steps at your convenience. Hopefully, they will help resolve the issue, and I look forward to hearing your update soon.

JeseminQH 515 Reputation points Independent Advisor

2026-09-01T03:21:30.8733333+00:00

Hi @Muhammad Sohaib,

It has been a while and I am writing to see how things are going with this issue. Have you had a chance to check the replies provided? Any update would be appreciated.
Muhammad Sohaib 20 Reputation points

2026-09-03T19:18:37.84+00:00
@JeseminQH

Are other users in your organization experiencing the same behavior? Yes

Have you tested Outlook on another device? If so, were you able to send the email successfully? same issue at other MAC Book 26.6.2 updated

Can you send emails successfully through Outlook on the web? Yes

Is the problem affecting all outgoing emails or only emails sent to this specific recipient? all outgoing emails with attachment

Have you tried using a different network, such as a mobile hotspot? Yes Multiple

Does the stuck email contain any large attachments? If so, what is the approximate attachment size? i am sending with in the attachment size limits i.e 10MB , 15MB in addition to when i convert Outlook for MAC New Mode to Legacy its provide me this ERROR in SYNC ERRORS "An Operation on the server Time Out. The server may be down or overloaded or there may be too much net traffic " Details "Email could not sent" ERROR Code -1001. This issue only reproduced when i am connected with internet using MAC OS. same MAC with same Microsoft Outlook client can send more than 20MB file easily.
JeseminQH 515 Reputation points Independent Advisor

2026-09-04T10:53:29.1566667+00:00

Hi @Muhammad Sohaib,

Thank you for providing the additional information. Based on the test results, this issue appears to be beyond the scope of troubleshooting available through the community, as it requires further investigation by a support team with access to the relevant Exchange server and Outlook connectivity logs.

Unfortunately, I do not have access to an on-premises Exchange Server 2019 environment in which I can reproduce and test this specific behavior. Therefore, I am unable to investigate the issue further from my side.

Since Exchange Server 2019 reached the end of support on October 14, 2025, you may wish to check with your Microsoft partner to determine whether they can investigate the issue and raise a support request through the Microsoft support channels available to them. Their ability to do so will depend on your organization’s support agreement and the services offered by the partner. For more information, please refer to Microsoft’s Exchange Server 2019 and 2016 end-of-support roadmap.

Alternatively, you may work with your Exchange administrator to upgrade to Exchange Server Subscription Edition or migrate to Microsoft 365 or Exchange Online. Moving to a supported platform would allow your organization to receive the appropriate technical support for issues of this nature. Upgrading to Exchange Server Subscription Edition (SE)

I am sorry that I cannot investigate this issue further as a fellow community user. I hope the information above helps clarify the available next steps.