I run Postfix as the mail transfer agent (MTA) on my server and saw a bunch of “error writing message: File too large” errors in /var/log/maillog as well as senders getting this bounce message:

I'm sorry to have to inform you that your message could
not be delivered to one or more recipients. It's
attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<example@example.com>: cannot update mailbox
/var/mail/example for user example. error writing message:
File too large

Ends up that Postfix has a mailbox_size_limit setting & since I didn’t have that set, it was set to the default 50MB. The user’s mailbox was near that limit so I up’d the default to 150MB in /etc/postfix/main.cf:

mailbox_size_limit = 157286400

then reloaded postfix:

service postfix reload

and all was good again.