You certainly can save all your messages to a USB stick or any other storage medium. HOWEVER NOTE! Depending on how you have configured your email client, the messages you wish to copy might not be local, and may actually reside on the mail server! It can also be a mix of here and there; for example, if you use an IMAP client, especially with GMail, you may have a local Inbox full of local downloaded messages, but the beast called "All Mail" resides on the Google server, and may contain duplicates of your Inbox messages, and even contain messages that you deleted long ago. Anything that shows up under the [Gmail] folder resides on Google servers. The following procedure creates local copies of the messages you wish to back up, regardless of their location:

1. So first... Start Thunderbird.
2. Then, create a NEW mail folder under "Local Folders". You may wish to create sub-Folders within it to preserve the organization within your backup.
3. Then, select and copy (or drag and drop) all of the messages that you wish to preserve into that folder and/or the sub-Folders that you created.
4. Then figure out where your "Local Folders" are stored: While still in Thunderbird, click on Help --> Troubleshooting Information.
5. Scroll down to "Profile Folder" and click the "Open Folder" button. An Explorer window opens and displays a lot of files.
6. Open the folder named "Mail". Then open the folder named "Local Folders".
7. The backup mailbox folder that you created will be in here. Assume that your backup folder is named "MY_BACKUP". There are 2 possibilities:

• If you did not create any subfolders, then you will see 2 files: MY_BACKUP, and MY_BACKUP.msf. Copy both of these to your USB stick (or other medium).
• If you created subfolders, then you will see 1 directory: MY_BACKUP.sbd. Within that directory, you will see the sub-Folders that you created, as described above (FILE and FILE.msf). If there are sub-Folders below this level, they will follow the same naming convention. You could either copy the entire MY_BACKUP.sbd directory to your USB stick (or other medium), or individually copy the folders.

BIG CAVEAT: These backed up messages will not be usable because they are large sequential message stores in RFC-5322 (affectionately known as "mbox", or RFC-822) format. Images and binary attachments are all encoded using base64, and often the text is encoded too. Therefore, the backup will be no use to you unless you have a way to read it! Since Thunderbird knows how to read this format, you might as well leave the messages in Local Folders, and keep Thunderbird installed, so you can continue to refer to them if you need to, but retain the USB media as an archive copy.

-GB

I need to delete my email account

Why?

it is crucial that I keep all the emails I have received and sent for the past 2 years.

It is recommended to backup the entire Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Whether additional steps are needed prior to backing up the profile depends on the account type (POP or IMAP). You haven't mentioned what type of account you have.