Error adding attachment

I’m having an intermittent issue where an image attached to an email is not included in the resulting post.

I get two emails in error. One that says:

“There was an error adding the attachment: filename.jpg”
and another that says:
“Failed to add image media file: filename.jpg”

Both emails state: “Sorry, this file type is not permitted for security reasons.”

Now here’s the catch: I’m only getting these errors when Postie is trying to import a large number of posts at the same time. (Typically 10 to 30 different emails and images within a short period of time.)

I realize this is likely not what Postie was designed for, but it’s an occasional requirement in my use case.

I’ve tried the following without success:

• Setting WP_MEMORY_LIMIT anywhere from 60 to 1024 MB.
• Restricting the number of emails imported at a time to 5, while checking for new mail every 30 seconds

While I could likely limit the number of emails imported further, importing them is somewhat time sensitive.

Any assistance is appreciated.