But my I don't have an @iCloud.com account. My Apple ID is @Mac.com
I presume you are discussing mail on a Mac? If you have an @mac.com address you will also have an @icloud.com address. The legacy addresses @mac.com and @me.com were ported to the successor platform when iCloud was commenced. I have one of these old addresses and Apple IDs in that format but I prefer these days to quote my @icloud.com address.
You can select @icloud.com on your outgoing mail or even select it as your default mail address in Mail Preferences > Composing > Send new messages from:
As to your recipient's junk mail folder - that is their responsibility to correct. Other than use an @icloud.com address with which you report success, there is no action you can take on your devices to regulate the behaviour of their email client. They will have to change their junk mail filters.
Have you been able to analyse any pattern for your mail going missing - would it be for customers of one ISP? To Windows users? To people behind a corporate server or firewall?
I would add though that you might be well served to select the option in Mail app on a Mac under Edit > Attachments > Always send Windows-Friendly Attachments.
The fact that you work in film production doesn't put you in a special category regarding email! Errant email messages would be a problem for anyone.