Hello,
Thanks for the question about Apple Mail issues when running on the current version El Capitan (10.11.5). I upgraded to El Capitan on my MacbookPro and the Mail function has been working with no problem for me. I looked at some of the forums, and the issues being reported are pre-11.5 version. You can see a list of possible issues in this article. I hope this helps to answer your question, please let us know if you require any further assistance.
Regards,
Arnel C.