Transferring email data to a memory stick from your mac is a relatively simple process. Email files, like any other file, can be transferred and stored to a memory stick and they don't take up that much space. Depending on what you plan on doing with the files, you can open them on any other computer. The files will be read only, like a text file, unless you log into your email account using mac mail on a different computer.
Plug your thumb drive into the USB port on your Apple computer. If you own an iMac, the USB ports will be in the back. Notebook computers have them on the side.
Open the mail application on your computer. If you don't know how to open the mail application, click on the finder icon on the top right of the desktop. A window will open and there will be a gray search bar on the top right. Type "mail" in the search bar and click on the mail icon that shows up.
Click on the finder icon once again if you haven't already to open another window. The USB port will show up on the top left of the window. Click on the icon to bring up the drive in the window.
Move over to the mail application once again. You will have all your messages loaded on the right. If you have more than one mailbox, select the correct one on the left by clicking on its name on the top right. Find the correct messages you want to move by either scrolling down or searching for the keywords in the top right search bar.
Select each message you want to move by clicking on them. You can select multiple messages at once by holding down the command key. If you select a message by mistake, click it again while holding the command key do de-select.
Drag each selected message from the mail window into the finder window you opened before. This will copy each message onto the memory stick while leaving the original copy in the mail folder. Remember that you can move one file at a time or multiple emails.
Click the "Eject" button next to the memory stick icon in the window. It is on the left hand side. This will make sure no files are corrupted by safely removing the stick from the computer.
Things You'll Need
USB memory stick
When you drop messages onto the memory stick, any attachments will be copied as well. Find the size of each mail message by selecting the message and holding command and the letter “i”. This will open an information window.