I would suggest you check a few things out about your own IP. There are lots of things you can do tim improve your deliverability.
You also dont mention whether you have done any inbox placement testing, a better indicator than open rates.
You will want to check your IP reputation and see if that is causing you any problems.
AuthSMTP and Sendgrid are not the only options, you can try smtp.com
Also have you made sure that your Authentication is configured correctly on your own server SPF records, DKIM, DK....
best
Andrew
http://emailexpert.org