Mailbox Rule throws "Invalid base64 string for MIME content"

Question

Mailbox Rule throws "Invalid base64 string for MIME content"

There are several threads for this issue, but they all appear to be closed with no clear resolution.

I encountered this problem today with an Inbox rule similar to the following:

However, when attempting to apply the rule I was presented with a popup containing the error "Invalid base64 string for MIME content"

I stumbled across a potential workaround, changing the actions similar to:

I am still monitoring/testing, but all indications are that this change does essentialy the same thing as a move, but the copy (instead of move) operation has not yet not thrown the "Invalid base64 string for MIME content" error.

Hope it helps someone out.

MacBook Pro (M3 Pro, 2023)

Posted on Jun 18, 2025 10:03 AM

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Answer 1

Jun 19, 2025 07:57 AM in response to Bradley Morgan

I was able to create a server-side rule that achieves the desired result.

In my case this is MS Exchange / 0365, using Outlook Web App to create an inbox rule. Not ideal, and may not work for all email platforms, but if your provider offers a web/server-side client or configuration app, you might be able to define the rule(s) there.

(In my experience/opinion, server side rules seem to be more reliable since they are triggered earlier in the route and stored centrally, which is arguably more efficient and avoids fragmentation across multiple clients.)

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Answer 2

Jun 18, 2025 11:10 AM in response to Bradley Morgan

Upon further review, after monitoring for some time, this workaround did not resolve the issue. My apologies. Perhaps it still might be worth noting.

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