[kuczmama]

You can try sending it to yourself from different email providers and check to see if it went to spam. For example I created a gmail and outlook account and sent emails to it from my hosted email server, once I had things set correctly I was able to validate the emails I was sending weren't going into spam.

[extra88]

How many different hosts are you going to create accounts on and send test emails to? Are you going to check the MX records for grandma's email to figure out who's hosting her email to test with that one?

Testing is not a one-time thing, the hosts keep changing their rules and if your ip is close to a spammer's that could change the treatment of your email as well. I sent email from my own domain address via an authenticated university SMTP server for years without hearing of any problems. Then this year, family members using Gmail started finding my messages in the spam folder. My best guess is Google started caring about the lack of an SPF record for the SMTP server, associating it with my domain but there's no way to know.

This is all way the advice for most people is: Don't run your own mail server, it's too much work and less reliable.