My 2012 Macbook Pro hasn't had these problems, but my partner's 2010 iMac is so plagued when she tries to use her SMTP function with Cox Cable.
Using Yosemite 10.10.3, her IMAP access to Cox worked faultlessly. She could easily receive Cox email transferred from Cox's Webmail page. (She still can. IMAP is no problem.)
But her Send Mail authentication method was constantly being changed to MD5 Challenge Response, rather than Password, which resulted in her going "Offline" each time. We found that once we changed the authentication method from MD5 Challenge Response to Password, and entered the proper password, she stayed online and could easily send email via Cox.net. Annoying but workable.
That was under 10.10.3. Yosemite 10.10.14 has opened a whole new can of worms. Sorry I didn't know about it before I installed 10.10.4!
My partner can no longer use Apple Mail to send mail via Cox. She enters her password, exits the SMTP editor, and voila! ... she's "Offline" again, while back in the SMTP editor, the password has disappeared. However, when I replicated her account on my MBP, everything worked fine. We're still not clear why her iMac has these problems and my MBP doesn't, but it's probably not a Cox issue.
Cox, however, is taking it seriously and escalating our trouble ticket to a higher-tier. The agent said, "We've heard it before." Probably nothing they can do about it, but we're glad they're trying. None of the solutions proposed in this forum so far has worked, so I'm setting her up with direct access to Cox's Webmail page via a Chrome bookmark.
Fortunately, my partner's Apple Mail still accesses IMAP (incoming mail) from Cox, so after she reads it, she can reply using another ISP's mailserver. In the end, she may prefer to switch all of her email needs to another ISP she favors. Which is why Cox is concerned.
We wish we could say the same for Apple. Typically, it's quiet on the issue, hoping I suppose that if not enough people are crippled in this fashion, it can ride out the complaints until OS 10.10.15 or 11.0 become available ... and then do the same thing all over again.
Apple Mail is regularly embellished -- Notifications, new layout, etc. -- but not mechanically improved upon. I'm now looking into Thunderbird and Air Mail, each of which has peculiarities but at least work.
PS I would seriously NOT disable Use Secure Authentication or use an unsecured port like 25. That's an open invitation to a crook to highjack your email account.