Sent email showing displaying as base 64 in "Mail" app

I've seen the issue with the inbox and quitting and restarting the Mail app gets the emails to show properly. This appears to be a bug in Mail, but I have only seen it with office 365 email accounts

I have 3 users with this same issue. quitting the mail app does get the email to appear correctly or waiting a few hours. it happens whenever these 3 users either email each other or email themselves. all 3 users were on local exchange and then 2 were moved to o365. apple had me delete the mail app and reload but the problem came back. Also, it still occurs after migration to a new iPhone when restoring iCloud backups/transferring data. It also happens on these users iPads.

Have you been able to find a solution? I've been scouring the internet for solutions. We have a couple of end users who have the same issue from their iphones. The problem seems to occur when a sent message is copied to the iphone sent items or if the user BCC's themselves from the iphone. The external recipient receives a normal message, but the message in the iphone sent items is not encoded properly. We see the following content at the top of the message in the "bad" emails in sent items.

Content-Type: text/plain; charset="utf-8"

Content-Transfer-Encoding: base64

Hello Skinnelli,

To help us get a better understanding of the issue, we’d like to ask a few questions.

1. Does this happen only with emails sent to a specific contact, or does it happen with multiple contacts?

2. Are any error messages displayed when sending the email? If so, what does the error say?

3. Is the issue only with the sent emails? In other words, do received emails display as expected on the device?

4. Do the emails ever fail to send, or do they appear in the Outbox  for unsent email?

The above details will help us plan how to proceed.

If emails aren't sending as expected, please check out this link, which has some information that can help with issues sending email: If you can’t send email on your iPhone or iPad

Before you begin

There are a few things to keep in mind and check:

* When you make an iOS or iPadOS backup in iCloud or iTunes, it backs up your mail settings, but not your email. If you delete or change your email account settings, previously downloaded email might be removed from your device.

* Make sure that your device is connected to the internet.

* Check with your email service provider to find out if there's a service outage.

* Look for the Undo Send button in your Inbox or list of mailboxes. If Undo Send is available, the message has not been sent.

* Learn what to do if you can't access your iCloud Mail, or you can't send and receive messages with your @icloud.com email address.

Thanks!

Thanks for reaching out again, Skinneli. As the issue remains, we'd recommend reaching out to Apple Support to look into this further: Get Support

Regards.

Thanks for replying, Skinnelli, and that additional info, it really helps! It sounds like things were still being synced across the different servers involved, so glad to hear that you figured out what the behavior was that was causing this and how to adjust that behavior.

Have a great day and thanks for using the Apple Support Communities!