I think so. In desperation I turned off the spam filter and the email is now working. Not entirely satisfactory as I don't understand why.
I did notice an email from "Cron Daemon" which said "Couldn't read directory /home/e-smith/files/users/rsmith/Maildir/;junkmail/new at /usr/bin/spamfilter-stats.pl line 455, <ORIGINAL> line 103."
I did a search on the topic of emails being sent to problems directory and an old entry recorded a similar problem because of a full directory. I guessed that a spam directory was full ... but it was only a guess.
Any ideas?