In order to avoid sending your emails to spam, you should comply with the requirements of email providers. Comply with the following:
Base should be honestly collected and subscribers should be voluntarily subscribed to your mailings. Do not send letters to those who don't want to get them. It is strictly prohibited to buy subscribers base or to use somebody`s base. Mailing to such emails will cast a shadow over your reputation as a sender and will provoke indignation and high complaints of recipients.
Setup SPF, DKIM and DMARС signatures for your domain. They help email providers to identify you, otherwise, your emails will be sent to spam. Instruction for setting up these signatures is here.
Users should always have an opportunity to unsubscribe from your mailing at any time. Unsubscribe button should be prominent and working. Add the unsubscribe button to each email. If the visual editor is disabled, use:
<unsubscribe>Unsubscribe in one click</unsubscribe>
Monitor the hygiene of the base. We recommend to clean the base periodically by means of email validation services. They will help to find out spam traps, inactive and bad addresses.
Email validation services:
Short URLs are often used by spammers, that's why email providers will send a letter with such link to spam. Check the absence of short URLs in templates.
There are stop words that you should not use in the subject. For example, "FREE", "Sale!" and so on. Also, many exclamation marks should not be added to the subject.
Postmasters help in analyzing bulk mailings. Using them, you can find the details about the deliverability of mailings, audience response, spam metrics and so on.
Email providers track the presence of your IP or domain in blacklists of senders with bad reputation. Do not forget to check the blacklists for your data. Use https://mxtoolbox.com for this.
Test the letter in the service https://www.mail-tester.com/ before sending. It will show the probability of going your mailing into Inbox.