Emptying your mailbox
Whatever the messaging system you are using (Thunderbird, Outlook or the webmail of your Internet service provider), the capacity of your mailbox is usually limited.
Thus, if your mailbox is full, it is necessary to delete messages to free up space. To do this, organize your emails by size and start to remove larger messages by selecting the desired mail and pressing the Delete button.
A good thing is to open your Recycle Bin and empty it! Remember to clean up the sent items, drafts, because the allocated space is shared between all your mail folders.