Go to SPAM folder of recipient mail server (for example gmail.com)
Select your message -> select 'Show original'. Then you will see source code of your email message with all SMTP headers.
See at string:
If 'Score' greater then 2.00 your messages will be bounced.
The lower 'Score', the more likely your letter will get into Inbox.
Score calculated from sum of each 'Symbol' scores.
Score of each 'Symbol' string placed in rounded brackets.
'Symbols' are quality metrics of your messages. If your message will be pretty good - this message will be released by our SMTP gateways and will be placed in "Inbox" folder in recipient mailbox (not SPAM folder.
If your message is dirty bad - this message will be filtered by our SMTP gateways and you will be receive this bounce message (Spam alert).
This way helps us to protect reputation of our IP's and yours domains.
You can enfluence on 'Score' (reduce) metric by re-writing your email template (body, subject, headers) or adjusting domain settings (DKIM, SPF, DNS RBL delisting).
When 'Score' will be lower then 2.00 - your mailings will be released.
Start from 'Symbols' with max scores, they have max influence.