One long standing issue with OS X 10.9 Mavericks is that it’s version of Apple Mail can be problematic when setup with multiple accounts, especially when Gmail accounts are in the mix, even more so when there is a large amount of stored email involved. The versions of Apple Mail included with previous OS X releases are considered more stable with complex email setups like this.

It was hoped that OS X 10.10 Yosemite’s Apple Mail would resolve this, but unfortunately that doesn’t appear to be the case. The following report states “The short version is that Mail is (for better and worse) about as reliable as it was in Mavericks”: http://tidbits.com/article/15305

For these reasons, unless you have just one email account, I would not recommend using Yosemite’s Apple Mail. The same is true for Mavericks. Outlook is still my preferred Mac email client.