serious issue in classified theme emails

@Mark,
I hope you would respond the query.

whenever users get registered or add listing on classified theme emails goes in to their spam folder instead of inbox.i have tried following but none of them workerd.

1. set up email in c panel and used in theme still email going in customer spam folder.
2. set up gmail account and used in theme still email goes in customer spam folder.
3. changed the style of the email and did everything so email does not look spam type email but not worked.
4. contacted to hostgator and they came up with following response:

when i directly send email by webmail or web client then there is no issue
(9:23:54pm)Jeremy O.:That strongly indicates an issue with the coding or configuration of your wordpress theme, that’s something that we’re not able to correct for you, coding issues are outside of our scope of support. I sincerely apologize for any inconvenience this may cause.
.
ok but what about blacklisted ip?
(9:27:44pm)Jeremy O.:if you’re able to send through webmail or with a client then a blacklisted IP is not the issue.

Now I am unable to launch my website due to this issue because I will have to stick to the computer almost all day to send emails to customers for their listing.

I would also like to receive suggestions from others if they could help.

thanks.