My favourite testing tool for email delivery issues is https://www.mail-tester.com/ which provide in-depth test results instantly. If the mail server IP address is listed on any spam databases, it will display, which means you will have to go to each database website and request to remove the flagged IP
Example:
SPF record
An SPF record as an SPF TXT record is a record that is part of your domain’s DNS. Contains a list of all IP adress with an SPF record in place, you protect your email domain against spoofing and phishing attacks. This is done by letting the world know which servers are authorized to send authenticated email on your behalf.
When a sender tries to hand-off email to an email “receiving” server for delivery, the email server checks to see if the sender is on your domain’s list of allowed senders. If it is, then a link has been established between the piece of email and the email domain.
Learn more about SPF records here:
https://support.google.com/a/answer/33786?hl=en
DKIM or DK signature
DKIM is a process that verifies the transmission of domain names using e-mail messages through cryptographic authentication. This is accomplished by inserting a digital signature into the message header, which is later verified by the receiving host to authenticate the sending domain.
Example:
Content
Body of your message critical, make sure they are well written without spammy words. Always have a plain text version to support if HTML version is not accessible to end-user. Avoid using shortened URLs in your messages because spammers love to use redirects. Here is a quick tool to test your SpamAssassin score;
https://spamcheck.postmarkapp.com/
Email Signature
Make sure there are no broken links in your email signature, no spammy / flagged words. This could result in mail servers flagging your email as suspicious, resulting your sent mail to deliver in to SPAM / JUNK folder of your receiver. If your email signature has any images, make sure they have a meaningful ALT tag.
