After a while I got roundcube login working and then spent some more time getting it to send and receive. The main issue was permissions for the dovecot and exim directories. I used the following commands to get it working:
for dovecot permission
sudo chmod +x /etc/exim/domains/
sudo chown -R dovecot:dovecot /etc/exim/domains/
sudo systemctl restart dovecot
for exim permission
sudo chown -R exim:exim /etc/exim/domains/
sudo service exim restart
for dovecot permission
sudo chmod +x /etc/exim/domains/
sudo chown -R dovecot:dovecot /etc/exim/domains/
sudo systemctl restart dovecot
for exim permission
sudo chown -R exim:exim /etc/exim/domains/
sudo service exim restart