Dustin Lammiman on "[Plugin: WP Mail SMTP] Authentication Issue (gmail smtp)"

I had this plugin working great with my Google Apps email for about a week but it suddenly stopped working. Relevant errors when I try to send a test email:

string(370) "SMTP Error: The following recipients failed: sampleemail@test.com
SMTP server error: Please turn on SMTP Authentication in your mail client, or login to the
550-IMAP/POP3 server before sending your message.  mydns.server.here
550-(www.mydomain.com) [webshost.ip.here]:55411 is not permitted to relay
550 through this server without authentication.

SMTP -> ERROR: RCPT not accepted from server: 550-Please turn on SMTP Authentication in your mail client, or login to the
550-IMAP/POP3 server before sending your message.  mydns.server.here
550-(www.mydomain.com) [webshost.ip.here]:55411 is not permitted to relay
550 through this server without authentication.

SMTP Authentication is on in the plugin settings. Are there any particular settings I should check with my web host?

http://wordpress.org/extend/plugins/wp-mail-smtp/