To change or adjust the delays in between messages for a sending email address can be done in the Account menu.
Start by clicking Account in the bottom left of your screen.
Select “All email accounts”
Click the two dots on the right side of the email you want to adjust.
Select “Edit”.
Change the value in the third box (“Min delay”) to the desired amount.
Note: This is not the exact amount of time between each email. This is just a minimum delay.
Click “Save” in the bottom right corner of your screen