I have discovered an issue with either the Mavericks OS or the native Mail App on my MacBook Pro with Retina. The issue didn’t exist with Mountain Lion as I have done the email process for several months successfully on Mountain Lion.
I have to send out distribution emails to an organization regularly which involves 80+ email addresses. I would use the native Mail App on the MacBook Pro and type in the message. I usually copied and pasted email addresses that were in a column in an Excel spreadsheet to the CC or BCC field in the message. Now, when I do that in Mavericks OS, I get a WARNING and it will not SEND the email. To recover any control of the MacBook Mail App, I have to press “esc” button. The only work around in the Mail App is to put a comma delimiter or separator between the email addresses which were pasted in from the Excel Spreadsheet column. This is rather tedious for 80+ email addresses in the message. However, when I did that once, the email then went out just like it used to do in Mountain Lion. I could make all the 80+ email address contacts and then make a group on the MacBook Pro Address book. BUT the email addresses often change so I would have to keep them up on the master spreadsheet and separately on the MacBook Pro Address book. I had rather keep them in one place instead of two.
This issue should be easy to replicate with this description.
I have also found that IF I abandon the use of the Mail Client App on the MacBook Pro and go to Gmail on the internet, the same process works fine using the Web Gmail Client. It immediately recognizes the addresses when pasted into the address fields without having to have any kind of delimiter between the email addresses.
Maybe Apple will fix this but I understand that it was a design change by their chief engineers. If you have the same problem, hopefully you will still have a webmail option with your email account service.