It is definitely frustrating when messages your recipients want end up in their spam folder. There are a lot of different factors that contribute to whether or not a message makes it to the inbox, but they all come down to reputation.
Years ago, reputation was just assigned to your IP address. These days, your reputation is being calculated on hundreds or thousands of signals. The primary factors that determine your reputation are your IP address, your domain, and your content.
IP Address Reputation:
Inbox providers are inherently distrusting of IP addresses they haven't seen. It is critically important that you warm up new IP addresses slowly. It is also important to avoid large spikes in volume. Take care to ensure your sending volumes are as consistent and predictable as possible.
Even properly warmed IP addresses can generate a bad reputation. Low engagement, bad list hygiene, and high spam report rates can all drag down your IP reputation and cause messages to land in the spam folder.
Your domain builds a reputation in the same way your IP does. Just like IP addresses, "new" domains are untrusted by inbox providers. New domains should be warmed up slowly so that spam filtering algorithms can get to know you and trust that you aren't a spammer. Inbox providers are also looking at engagement, list quality, and spam reports to determine good domains and bad domains.
Your content is the third major factor in filtering decisions. ISPs are constantly fingerprinting content and comparing those fingerprints to engagement results and spam report rates. If your content is consistently generating high spam report numbers and low engagement, it will begin to generate a bad reputation.
Text to image ratio is also worth considering. If a message is just one big image, a spam filter can't tell if it is spam or not. A good rule of thumb is to include at least two lines of text for each image in your email messages. We recommend an image to text ratio of 30:70 or 40:60 to avoid the spam folder.
Make sure your unsubscribe link is functional and prominent. It may seem counterintuitive, but if recipients can't find (or don't trust) the unsubscribe link they will use the "Spam" button. An unsubscribe is always better than a spam report.
Other things to consider:
If your messages are landing in the spam folder, dial back your sending frequency. Sending a bunch of messages that are landing in the spam folder isn't doing yourself any favors. Send less mail until the problem gets better.
Send to engaged users:
If your messages are landing in the spam folder, you need to target your recently engaged recipients. Sending mail to people who aren't opening is a quick way to sink the reputation of your IPs, domains, and templates. A general rule of thumb: Don't send to anyone who hasn't opened or clicked one of your messages in the past 6 months. 2 to 3 months may be more appropriate for senders with a daily mailing frequency.
Make sure you have domain authentication, reverse DNS and link branding in place in your SendGrid account provided by MassMailer. This will allow you to take more ownership of your own reputation rather than sharing a reputation with thousands of other senders using the same domain in their messages.