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I'm in the process of moving around 30.000 emails from an Office 365 account to a free Gmail account. I tried several ways to do this:

  1. Copying emails in Outlook 2016 for Mac from the original folder in the Office 365 account to the destination folder in the Gmail account;
  2. Copying emails in Thunderbird for Mac from the original folder in Office 365 account to the destination folder in the Gmail account;
  3. Having Gmail fetch the messages to the Office 365 account root folder by POP access.

In all cases, for older emails previous to 2007 or so, the messages in Gmail appear only in text format and with the message: "This message is in MIME format. Since your mail reader does not understand this format, some or all of this message may not be legible."

Is there a way to move the messages and have them show up in html in Gmail?

  • Connect the new account as IMAP, connect the old account as POP3 and download copies of every email on the server, then copy each email to the server. This will take awhile, but ANY approach, would take awhile since you have 30,000 emails. Connect both accounts using the same client. – Ramhound Apr 25 '16 at 20:53
  • If you forward a few of these messages does that change the behavior? That is, if you pass it thru an SMTP server over to GMAIL, does it render properly? – Jesus Shelby Apr 26 '16 at 13:35
  • If I forward to a gmail address, it renders properly indeed. – iCosmo Apr 29 '16 at 12:36
  • Ramhound, I've tried your tip. However when downloading messages through POP, they also come unformatted. I've tried this with new accounts both in Outlook 2016 for Mac and also in outlook.com. It seems that if the old messages leave Office 365, they loose the html... – iCosmo Apr 29 '16 at 12:54
  • This is your issue: Email recipients receive a Winmail.dat attachment from Entourage for Mac users in Office 365 support.microsoft.com/en-us/kb/2022993 – Sun Apr 29 '16 at 14:59
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Using Outlook 2016, use the Clean Up Conversations features. That'll reduce your 30k to a much smaller set. Add your Gmail account to Outlook 2016. You may need to turn on "Access to less secure apps" since Outlook does not utilize the latest security protocols. Then you just drag the e-mails over to the Gmail folder.

  • This is what I've done, it does not solve the format problem for older messages. – iCosmo Apr 29 '16 at 12:38
  • You can try reading this thread: community.office365.com/en-us/f/158/t/57178 -- Older versions of Outlook for Mac were not compatible with Outlook for PC if you can believe. You might have to fire up a Windows VM or use a Windows PC with Windows Live Mail, Outlook Express (on an older Windows version like XP), or Thunderbird. You'd have to export the existing e-mail to a format that one of those programs understand, and hope the conversion fixes the MIME error, and transfers to Gmail okay. – Sun Apr 29 '16 at 15:02

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