Most MUAs available on Unix-style systems still support local mailboxes, but it's not necessarily obvious how to access them.
Create a new mail account. Set your email address as
USERNAME@localhost. Disable "Look up mail server details based on entered email address". Under "Receiving email" set your server type to "Standard Unix mbox spool file" and set the file location to
/var/mail/USERNAME. Under "Sending email" set your server type to "None".
Geary does not currently support mboxes. It requires that every mail account has an associated SMTP and IMAP server, so you will have to configure these locally.
Cancel any email account wizards, then access the account settings (AltE to open the “Edit” menu, then “Account Settings”). From there, click on the “Account Actions” drop-down and select “Add Other Account...”. Check that “Unix Mailspool” is selected and click on “Next”. Fill in an identity — the local part of the email address will be used by default as the spool name (the file in
/var/spool/mail which contains the email you want to read) — and click on “Next”. Skip the “Outgoing Server Information” page by clicking on “Next”. Check the account name and click on “Next”. Finally, click on “Finish” and review the account settings. You should then be able to retrieve the email from the local mailbox.
In many cases, MUAs will move email from local mailboxes to their preferred storage, which means you won’t be able to read email using another MUA afterwards.
mutt can be told not to do this, I’m not sure about Thunderbird & co.
To test, you can send a local email with
mail USERNAME@localhost. Fill in the prompts as follows
$ mail USERNAME@localhost
Cc: <leave blank, hit enter>
Subject: test subject
test body <hit enter>
You can confirm the mail has been received by typing
mail. Hit Q to quit the text-based mail client. Now check your inbox via your preferred MUA GUI.