Some email programs create a folder called Sent Messages, so when we migrate your email to the new system, there can be a Sent folder and a Sent Messages folder. When updating the settings, sometimes your email application can start looking at the Sent folder instead, so your Sent seems empty.

There are 2 solutions:

1. Move messages into Sent

Move all messages from Sent Messages into Sent. In Mac Mail you can simply select all messages within the Sent Messages folder and drag and drop them into the Sent folder.

2. Set MacMail to use the Sent Messages folder again

Apple Mail defaults to a sent mailbox/folder named "Sent Messages". If the option to "Store sent messages on the server" is active it will create a folder called "Sent Messages" on the server and use that folder to save copies of sent messages. This conflicts with many other applications which use the folder "Sent" instead.

Here's how to change the default sent mailbox using Apple Mail on macOS 10.12 Sierra:

1. Open Mail and choose the "Mail" menu in the upper left. Select Preferences.
2. Choose the "Accounts" Tab at the top, then select the account you wish to adjust in the list on the left
3. Select the "Mailbox Behaviours" tab on the right
4. Under "Sent Mailbox" use the drop down to select the folder called "Sent". Be sure that it is a folder underneath the header indicating your server-side email account and NOT "On My Mac" (that is a local folder). 

Here's how to change the default sent mailbox for Apple Mail on OS X versions 10.11 and earlier:

1. Select the "Sent" folder on the server (in the folders pane on the left) by clicking on it once. It should *not* be at the top of the page next to Inbox/Drafts/Junk/etc, nor should it be under "Local Folders" or "On My Mac". It will be found under a heading indicating that it's a server-side folder. 
2. Chose Mailbox from the system menu at the top of the screen
3. Choose "Use this Mailbox as" > and then choose "Sent"

Note that if you are unable to select "Sent" in step 3 because it is greyed out (disabled), it's because you have not selected a mailbox -- start again, paying close attention to Step 1.