1. Email content
Email messages are marked as SPAM when the anti-spam algorithm on the recipient's server marks the email with high enough spam score. Each server uses different algorithm, so for example @gmail.com can mark the email as spam and @yahoo.com doesn't have to and vice versa.
The score email gets is based mainly on the subject line, the content, the sender's SMTP server (server used for sending email), SPF and DKIM records in DNS etc.
The subject line and the content's shouldn't resemble spam email, they should match each other, don't look suspicious etc.
For example we found out, that Gmail blocks emails that have the word "proposal" twice in the subject.
Here are some tips how (not) to write subject lines: http://kb.mailchimp.com/article/best-practices-in-writing-email-subject-lines
A short excerpt from this article: you shouldn't use for example these words in the subject: Free, Help, Percent off, Reminder
2. Set up your SMTP server or authenticate your domain via DNS
A) SMTP server
You can also decrease the spam score if you use your own SMTP server that you use usually for sending emails, instead of using our shared server. You can set this up in My account / SMTP server. You should be able to find your SMTP server info needed for this in your desktop email client (e.g. MS Outlook).
To increase the deliverability also make sure your SMTP server itself is set up correctly - usually you need to add SPF, DKIM or Return-path records to your DNS - your email provider (Google suite, Office 365 etc.) should have instructions on how to do this.
Take a look at Gmail's answer to this question for more details: https://support.google.com/mail/answer/1366858?hl=en
B) or authenticate your domain via DNS
If you can't or don't want to use your existing SMTP server, please go to My account / Email & domain authentication and verify your email addresses and/or authenticate your whole domain (e.g. yourcompany.com) by adding DKIM and Return-Path records to your DNS - this is highly recommended, because it will greatly improve your deliverability! Currently we use Postmarkapp.com for sending these emails.