Last night about 2005 this message started appearing in my logs:

Jun 21 09:26:31 abc dovecot: deliver(www-data):
mkdir(/var/www/Maildir/cur) failed: Permission denied (euid=33(www-data)
egid=33(www-data) missing +w perm: /var/www)
Jun 21 09:27:31 abc dovecot: last message repeated 4 times

I was working on some php code but nothing to do with sending mail.
Its showing up in my log about ever 3 min.