I hope you are well.
If you are using Mac Mail on Yosemite you will notice that since you updated to Yosemite there is an SMTP setting that reads
"Automatically maintain and correct settings."
No matter how many times you set the SMTP server settings you will notice the same issue every now and then as Mac Mail decideds to correct our configuration as it sees fit. This often defaults to TLS and creates a syntax error when attempting to send mail and this is the reason for the SMTP server list pop-up. This syntax failure actually is a mismatch between the TLS and SSL settings that Yosemite and Mac mail keeps defaulting to. If you uncheck this box and config SMTP you will be dandy :)
Hope this helps Mac users.