When using G Suite Sync for Microsoft® Outlook® (GSSMO), here are a few things to note when working with messages in Outlook Mail.

Organizing Mail

  • See a message in two folders? The message isn't a duplicate. It just has two labels assigned to it in Gmail and therefore appears in each corresponding folder in Outlook. (Remember that folders in Outlook map to labels in Gmail, and a message can have multiple labels.) You can leave the message in both folders to find it in either location. Otherwise...

Don't delete a "duplicate" message unless...

  • Deleting a message from one folder in Outlook deletes it from any other folders, as well, which might not be what you want.
  • To remove the message from one folder but not any others, drag it out of that folder to Outlook's Archived folder (this is equivalent to removing the label from the message in Gmail). Or, Shift-Delete the message from the folder you want to remove it from. When you do either of these things, the message is still available from its other folders.  

Sending and Receiving

  • Sending large file attachments (over 3MB) from Outlook can take longer than usual with GSSMO. However, you can keep composing and reading email while the send/receive process completes.
  • Sending web pages from IE or Office can be delayed: If you send someone a web page from Internet Explorer or Microsoft Office using the "Send > page by mail" option, the message is sent immediately only if Outlook is open. Otherwise, it's not sent until the next time you start Outlook.
  • Clean reply headers may look different. Clean reply headers in email threads between Gmail users and Outlook users look different than in threads between Outlook users. This is because Gmail sends slightly different headers than Outlook.
  • Messages you send to yourself from a third party application such as Adobe Reader might be marked as Read in Outlook's Sent messages folder—even if you haven't opened the message from there. Such messages always appear as Unread in your main Inbox, however (until you open them, of course).

International Text

  • Emailing Outlook items with non-English characters in the Subject: If you send a contact (in Outlook format) that has non-English characters in its name field to another Outlook user, the characters won't display properly in the recipient's Subject field. Instead, the characters appear as question marks (???). The same thing happens if you forward a Calendar appointment with non-English characters in the Subject field.
  • Non-English characters might not show up in a custom From address: If you send a message in Outlook using a custom From address that contains non-English characters, the characters appear as question marks (???) to recipients, unless you also include non-English characters in the message's Subject. Learn more about custom From addresses.

Source: https://support.google.com/a/users/answer/158095?hl=en&ref_topic=29321