Why are my emails being treated as spam?

[#1]

This morning I was told by my local parkrun that my emails to them went straight to their spam folder. They found them by chance. This is a recent problem because, before amonth ago, my emails to parkrun had got to them without a problem. I told a friend who I had also sent emails recently, and he told me that he had found my emails in his spam folder too. As far as I know, these are the only cases but if my emails are going to spam folders how would other recipients know I had sent them?

Do I have a spam folder (where is it hiding - I don't know), so how do they know they have them, and why would they go to those spam folders to see if anything is waiting there?

Is this a Greenby problem or gmail problem, or what? I dont use gmail but the recipients might.

Any answers, please? Thanks in advance for any help

[#2]

Which SMTP server name or webmail address are you using to send ?

Which email operators are sending the messages to junk mail ?

if you can get hold of the email header / source of a few of the junked messages
you can investigate the exact reasons for the junk / spam status. 

E.g is there a FAIL for SPF / DKIM or DMARC or some other reason for downgrade to junk status.