Use this procedure to change Apple Mail's Default Sent folder.
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 that use the folder "Sent" instead.
For macOS 10.12 Sierra
Open Mail and choose the "Mail" menu in the upper left. Select Preferences.
Choose the "Accounts" Tab at the top, then select the account you wish to adjust in the list on the left.
Select the "Mailbox Behaviors" tab on the right.
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). If you do not see such a mailbox, you will need to create it by choosing the Mailbox menu, then selecting New Mailbox. For the Location field, you must select the correct account to create it under (not On My Mac), then set the name to "Sent".
For OS X versions 10.11 and earlier
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. For example: "Websavers" for a Websavers email account.
If you do not see a server-side heading in the folder list or if you do and there is no Sent folder there then you must create a new folder by choosing the Mailbox menu, then "New Mailbox". For the Location field, you must select the correct account to create it under, then set the name to "Sent".
Chose Mailbox from the system menu at the top of the screen
Choose "Use this Mailbox as" > and then choose "Sent Mailbox"
To configure the Default Sentfolder on an iOS device
Go to “Settings”
Choose “Mail, Contacts, Calendars”
Select the email account.
Under IMAP tap on the “Account
Choose “Advanced” at the bottom.
Under “Mailbox Behaviors”, tap on “Sent Mailbox”
Ensure that the folder called “Sent” is enabled under the “On the Server” section and is the same one that is being used by macOS Mail.
Test the changes by sending a message from each device and showing that they all show up in the same place.