I am finding that emails sent by me to at least two other IDNet users are not being received due to very harmless reasons such as blank or spelling mistakes in the subject line, photos attached or inserted in the body of a message. This has been established by looking at their Webmail accounts to find the missing messages classed as 'junk'.
Similarly, I have missed messages until I disabled the spam filter on all my accounts but despite that, very very rarely receive genuine spam. I have suggested to my friends that they do the same! However, genuine emails are now occasionally received with **SPAM*** added to the subject line by some machine!!