Emails sent through SMTP on VPS are considered to be spam

During our business we have to make regular mailing to our clients: invoices, information emails, etc.

Previously we received and sent emails using mail server of our hosting provider. But as the number of clients increased, we have to order VPS and install our own SMTP server their for performing our mailings.

So, now we have default provider mail server for receiving emails, let it be business.com. We have email accounts like info@business.com, etc. We use this mail server to receive emails and manage our email accounts.

And we have SMTP server which is running on VPS. We use this SMTP only for sending emails with From addresses like info@business.com. VPS has default DNS records created by provider, let it be IP.AD.RE.SS <-> ip-ad-re-ss.provider.com.

Mailings are made using either desktop email clients or custom Java-based application which uses JavaMail for sending emails.

The problem is that most of emails sent by us are placed in spam folders in clients email accounts. Clients have their email in Gmail, Yahoo, Hotmail, etc.

Could you please tell what is the most probable reason and solution of described problem?

Are there any service in Intranet where we can send test email and get an answer with description why this email could be considered to be spam?