Oliver,
Your URLs resolve as follows.
tinytwo.net = 66.218.79.155 (yahoo.com)
mail.tinytwo.net = 64.230.157.111 (sympatico.ca)
www.tinytwo.net = 64.230.157.111 (sympatico.ca)
Note the different IP adresses!
Are you hosting your web site on your SME Server or remotely at Yahoo?
mail.tinytwo.net is being directed to the same IP address as your web site.
I can not telnet to mail.tinytwo.net on port 25 so something is wrong with the DNS or redirection service. OR your ISP is blocking port 25.
Below is some DNS information that might help you figure it out. You may also want to check with your ISP to see if they are blocking any ports.
DNS Lookup Results for tinytwo.net
Host Type Value 
tinytwo.net MX 10 tinytwo.net 
tinytwo.net MX 10 mail.tinytwo.net 
tinytwo.net MX 10 mail.tinytwo.redirectme.net 
tinytwo.net NS ns9.san.yahoo.com 
tinytwo.net NS yns1.yahoo.com 
tinytwo.net NS yns2.yahoo.com 
tinytwo.net NS ns8.san.yahoo.com 
tinytwo.net A 66.218.79.149 
tinytwo.net A 66.218.79.155 
tinytwo.net A 66.218.79.147 
tinytwo.net A 66.218.79.148 
ns9.san.yahoo.com A 216.109.116.20 
yns1.yahoo.com A 66.218.71.205 
yns2.yahoo.com A 216.109.116.20 
ns8.san.yahoo.com A 66.218.71.205 
DNS Lookup Results for mail.tinytwo.net
Host Type Value 
mail.tinytwo.net CNAME mail.tinytwo.redirectme.net 
tinytwo.net NS yns2.yahoo.com 
tinytwo.net NS ns8.san.yahoo.com 
tinytwo.net NS ns9.san.yahoo.com 
tinytwo.net NS yns1.yahoo.com 
DNS Lookup Results for 
www.tinytwo.netHost Type Value 
www.tinytwo.net CNAME tinytwo.redirectme.net 
tinytwo.net NS ns9.san.yahoo.com 
tinytwo.net NS yns1.yahoo.com 
tinytwo.net NS yns2.yahoo.com 
tinytwo.net NS ns8.san.yahoo.com 
ns9.san.yahoo.com A 216.109.116.20 
yns1.yahoo.com A 66.218.71.205 
yns2.yahoo.com A 216.109.116.20 
ns8.san.yahoo.com A 66.218.71.205