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Outlook365 has determined that the email is too large to be sent

Data pubblicazione: Jun 30, 2026
Descrizione

Note: The information in this article applies to both Salesforce Classic and Lightning Experience.
This error occurs when a user sends an email through the Send Through External Email Services feature using Office 365, and the email includes an attachment larger than 3 MB. The error message displayed is:

Outlook365 has determined that the email is too large to be sent

This is a limitation of the Microsoft Office 365 API used by this feature, not a Salesforce bug.
Required Configuration for This Issue to Occur:
Send Through External Email Services must be enabled for Office 365:

  • In Salesforce Classic: Setup > Email Administration > Send through External Email Services
  • In Lightning Experience: Setup > Email > Send through External Email Services

The Email Attachments setting must be configured as:

  • In Salesforce Classic: Setup > Email Administration > Email Attachments
  • In Lightning Experience: Setup > Email > Email Attachments

The setting "Include as attachment up to Salesforce email size limit or as links if more" causes attachments larger than 3 MB to trigger this error when sent via the Office 365 API.

Risoluzione

 

The Microsoft Office 365 API has a 3 MB attachment size limitation for emails sent through Salesforce's Send Through External Email Services integration. To resolve this error, change the Email Attachment setting so that large attachments are sent as links rather than embedded files.
Steps to Resolve:

  1. Go to Setup.
  2. Navigate to the Email Attachments setting:
    1. In Salesforce Classic: Setup > Email Administration > Email Attachments
    2. In Lightning Experience: Setup > Email > Email Attachments
  3. Change the setting to one of the following options:
    1. "Always as links" — all attachments are sent as download links.
    2. "Include as attachment up to 3 MB or as links if more" — attachments under 3 MB are sent inline; larger attachments are sent as links.
  4. Click Save.

After making this change, users will be able to send emails with larger attachments through Office 365 without encountering this error.

Numero articolo Knowledge

000382374

 
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Salesforce Help | Article