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.

[#3]

Hello @PhilipHeyes .

I will try to answer your questions as best I can, but I'm not sure what you are asking, especially the second one.

(1) The SMTP server according to my Mailwasher software is "relay.plus.net". 

(2) The Parkrun email address on the email that got treated as spam was "[email protected]". I don't want to publish the email address of my friend just here in case it is a security risk - the parkrun address is readily available

(3) I may have misunderstood the third question. Do you mean I should get my friend to reply to my email that got spammed so that I can investigate it, rather than look at what I sent before it reached him?

Sorry if I seem a bit thick, but this is an area where I am particularly ignorant

[#4]

From the last comment we can see spam issues could well be due to SPF=FAIL because
Plusnet's relay.plus.com does not appear in the SPF list of approved senders for domain parkrun.com

https://www.nslookup.io/domains/parkrun.com/dns-records/

[#5]

To send via the Plusnet  hosted SMTP : relay.plus.net or via a Greenby hosted SMTP : mail.enmail.co 

The parkrun.com  domain would need entries like are seen here :

https://www.nslookup.io/domains/plusnet.plus.com/dns-records/


But would be easier / much preferable to replace SMTP relay.plus.net with an already defined SMTP server from Google.

E.g. if you collecting the emails from pop3 or imap.gmail.com then perhaps using the same credentials via smtp.gmail.com
are worth investigating.