Greetings PrimalUK,
Thanks for using the Apple Support Communities!
I understand that your iCloud mail has sporadic issues where you can’t send or receive mail. Right now, the account seems to be receiving mail with no issue, but sent mail is sitting in the Outbox.
It sounds like you have tried a lot of troubleshooting previously, but to continue troubleshooting this situation, I recommend reading over and working through the steps in the following article. The article contains a lot of great information and I do suggest reading over it all, but I am highlighting what I feel is the most relevant part for your situation.
- Make sure that you're connected to the Internet. Try to open apple.com in Safari on your iPhone, iPad, or iPod touch.
- Go to iCloud.com on a Mac or PC to see if you can receive email for your iCloud account.
- Tap Settings > iCloud and turn on Mail on your iPhone, iPad, or iPod touch.
- Tap Settings > Mail, Contacts, and Calendars > Fetch New Data and turn on Push. Then send an email to your account to see if Push is working.
- Tap Settings > Mail, Contacts, and Calendars > Fetch New Data. Then Tap your iCloud account and turn on Push. If you have multiple iCloud Mail accounts, you can use Push with one account at a time. Make sure that Push is turned on for your primary iCloud Mail account.
- Turn your device off and back on.
If you still can't send or receive mail, turn off each of the settings in steps 3, 4, and 5 above, then turn them back on.
Use these steps if you need help sending or receiving email in the Mail app on your Mac. If you still need help, contact iCloud Support.
Get help using iCloud Mail - Apple Support
Have a great day!