Start by changing your IP address, as it might have previously been used by a spammer and therefore be blacklisted by some spam filtering services. It couldn't be simpler to do as, in most cases, it involves nothing more than removing the power to your router, waiting for a minute or two and then powering it back up again.
I suggest finding out what your current IP address is first, which involves nothing more than clicking here:
https://whatismyipaddress.com
Then use that link again, after power cycling your router, to check that it's been changed.
Spam filters also watch out for certain words that are frequently used by spammers and then assume that an email with such words in their titles must be spam. So, for example, even though you're delighted that your nephew has just gained a first class honours degree at Oxford, it's unwise to include the word 'Congratulations' in the title of your email, as that might result in it being seen as spam by filtering systems.
The list here has been compiled to help bulk marketers get around such problems but it's equally useful to individual senders of emails:
https://www.activecampaign.com/blog/spam-words