Open Disk Utility in the Utilities folder inside the Applications folder and run First Aid on your Hard Drive. If that does not help let us know.
Thanks for the advice, it's a brand new MacBook Pro, 5 months old. I will do what you suggested. I highly doubt it will be the cause, I have also run the first aid utility before, with no issues detected. Will keep you updated.
Is your Mail issue similar to the one in this thread?
No success, reinstalled el capitan. No effect
Yes, if there is another way I would love to hear about it!