Email filters will stop your emails if the following occur
Write to one person, No CC, No BCC, No Additional recipients.
Address the person properly at the beginning of the email “Dear Clive” rather than “Hi m8”.
Avoid a deceptive subject line.
Give a proper subject, using common words, and avoiding all of the words you see in spam email (purchase order, pro-forma, invoice, licence).
Don’t send lots of emails to the same person.
Avoid very short emails.
Send from a normal email program on a normal computer, rather than another device.
Write your reply to email address at the bottom of your email or in your email signature.
Ensure your computer is set to use a proper character set (Avoid using something from a different country, like US-ACSCII, normal preference would be UTF-8).
Do not add one single image, either send more, or none.
Do not attach HTML files.