Hello,
I don’t send many emails but am also one of the people who started with MobileMe before iCloud came about. Do the recipients have you in their contacts? That may be needed depending on their email domains spam filtering. Ultimately, they would need to check with their providers. Not much can be done on the sender side.
If the recipients are also using Apple email domains this article will help them mark you as legitimate. Identify and filter junk mail in iCloud - Apple Support
“Filtering out junk mail can block a legitimate email from reaching your inbox.
If you feel that a legitimate email message was accidentally filtered, you can mark it as not junk so that any future messages from the same sender go to your inbox”
Alternatively, you may be able to add an @iCloud.com alias to see if it is also filtered. iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support
Using your @icloud.com email address - Apple Support
I hope this helps.