If you have an e-mail message that exceeds 25 MB, it will prevent the upgrade of your e-mail to Office 365. You can use Mac Mail to find e-mails over 25 MB to delete or save attachments to external storage. The following instructions apply to Mail 5 (OS X Lion) or Mail 6 (OS X Mountain Lion).
- Open the Mail program.
- Click on the Mail menu and select Preferences.
- Select the Viewing tab and make sure the option labeled “Use classic layout” is checked.
Note: You can uncheck the option when you're done deleting and saving e-mails over 25 MB.
- Typically, e-mails are sorted by the date received, with the most recent at the top. The next few steps, will allow you to sort by size.
- To show the Size column, right-click the column header bar and select Size.
- The Size column will appear showing the size of each message.
- Click the Size header to sort e-mails by size, typically largest to smallest from the top. Once sorted, you may have to scroll to the top to see the largest e-mail sizes.
- Open your large e-mails and save any attachment(s) that need to be saved by right-clicking an attachment and selecting Save Attachment. Next, specify where to save the attachment.
- Repeat this process for all e-mails that are 25 MB or greater.
- Once all e-mail attachments are saved, select and delete the e-mails that are 25 MB or greater.