Issue
When using Send an Email action with External email service and having a total size attachment of more than 20 MB, the workflow terminated with the following error:
System.Activities.Statements.WorkflowTerminatedException: Total size of all attachments (x MB) exceeds the allowable limit of 20 MB.
Error Code

Resolution
Reduce the total size of attachments up to 20MB.
Additional Information
Send an Email (External service) with attachments has a limit for the total size of all attachments up to 20 MB.
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