Hai frens,
first, just tell you mikrotik isn't mail server. But you can send mail from your Mikrotik machine. than you must put smtp ip address to it, don't put with dns name. or you have your own smtp-relay ip address
solution:
- ask to your internet provider or
- check from console like this. [e.g: you have smtp dns name smtp.your_isp.net]
/ping smtp.your_isp.net
or
/:put [:reso smtp.your_isp.net]
than press enter key button, you can see report ip address on your console.
reso same resolve command, but for console and you must use :reso
i like typing any script 4 caracters minimum..... 
i used this script Update SMTP & Backup scripts cos sometime my main isp change their smtp ip address [maybe for their security], than i must update first before mikrotik sending any mail.