Q: Can't send e-mails using iCloud address on either my iPhone or Macs ?

Hi guys and gals,

I can't seem to be able to send e-mails using my iCloud address.

No idea why that is. I don't get any error message, the e-mails 'get sent' without any fuss or ending in the outbox folder and they simply do not reach their destinations. There doesn't seem to exist any issues with Apple either. Perhaps my e-mails are being blocked, out of the blue (not by my ISP though, I'm having this problem with whatever network I'm connected to or using  internet data on my mobile). ?
 

Any help would be appreciated, thanks !