I've just upgraded to Mavericks from Snow Leopard as that is not supported with latest vulnerabilities etc. I checked the HD and that was showing possible errors so I go through the pain of changing the HD, rebuild snow leopard from Time Machine then download mavericks and install. That was a few hours to say the least. All was fine then Mail suddenly stops working and the server is sending back a 503 command error when sending emails 
Every other device can send emails through that account. But, get this, tried on the other mac which is still running Snow Leopard and it will send emails when I reset at the server, but if I then try to send an email from the Mavericks machine the Snow Leopard fails!!! Major fail, absolutely major! I have a business to run and spent hours now converting all the mail boxes to Thunderbird. I don't like apple support as for example I could have changed the faulty Seagate drive by 'popping down' to the mac centre with my 27 inch machine for free but that would be fun wouldn't it - NOT! So easier to fix it myself and buy the drive myself.
Thunderbird is not stable enough for business IME.
What a mess! Absolute bloody mess!
Thanks Apple.
Every other device can send emails through that account. But, get this, tried on the other mac which is still running Snow Leopard and it will send emails when I reset at the server, but if I then try to send an email from the Mavericks machine the Snow Leopard fails!!! Major fail, absolutely major! I have a business to run and spent hours now converting all the mail boxes to Thunderbird. I don't like apple support as for example I could have changed the faulty Seagate drive by 'popping down' to the mac centre with my 27 inch machine for free but that would be fun wouldn't it - NOT! So easier to fix it myself and buy the drive myself.
Thunderbird is not stable enough for business IME.
What a mess! Absolute bloody mess!
Thanks Apple.