Issue using TLS with Rainloop and OpenSMTPD

Question

On Ubuntu 14.04, trying to set up authentication for Rainloop SMTP, IMAP set up successfully with dovecot.

Image of Rainloop config.

Rainloop is configured for STARTTLS at 127.0.0.1:587, using short logins and authentication. OpenSMTPD config is as follows:

# This is the smtpd server system-wide configuration file.
# See smtpd.conf(5) for more information.

pki [url] certificate "[full chain cert location]"
pki [url] key "[private key location]"

table creds "/etc/opensmtpd/creds"
table vdoms "/etc/opensmtpd/vdoms"
table vusers "/etc/opensmtpd/vusers"

# To accept external mail, replace with: listen on all
listen on eth0 tls pki [url]
listen on eth0 port 587 tls-require pki [url]

# If you edit the file, you have to run "smtpctl update table aliases"
table aliases file:/etc/aliases

# Uncomment the following to accept external mail for domain "example.org"

accept from any for domain <vdoms> virtual <vusers> deliver to mbox
accept for any relay

vdoms and vusers files are available if needed, but shouldn't be the issue.

Rainloop is located on the same device as both the IMAP and SMTP services.

Testing through Rainloop fails, but a local test with OpenSSL on port 587 with StartTLS and credentials works. Squirrelmail and external mail clients work with IMAP but not SMTP.

No clue where to go from here, genuinely stumped as to why this isn't working, any help is appreciated.