Somehow people forget all about the Send File and treat it as an infinite buck for
storing old correspondence.
Systems have limits and that includes FILE SIZE. Sadly, a great many email programs
store messages in one file per email folder, eg the Inbox is one file, Sent folder is one file.
This clearly leads to a day in which the I/O to the Sent file will fail.
The only solution for this condition is to Archive Sent messages and or to delete
messages, say all messages from last year.
Outlook has a feature to Archive.
While discussing Email, Files and Systems, I might note that when you delete an
email message from the Inbox, it does not actually get physically delete but ONLY MARKED for deletion.
The Inbox remains the same size and it too will fill up. To actually delete email requires that the Inbox be COMPRESSED
In Outlook, this takes place with the archive and each fold (Inbox, Sent, Calendar, ...)
can have it's own archive criteria and schedule