If you use an email client for sending emails the default outgoing smtp server for otenet is mailgate.otenet.gr according to this.
From what i’ve figured out this acts as a gate that sends you to one of the smtp servers of Otenet.
The thing here is that Otenet smtp servers tend to get blacklisted every now and then and there is nothing you can do about it. The mails would bounce back from the server following this error.
This is the mail system at host medusa.otenet.gr.
I’m sorry to have to inform you that your message could not
be delivered to one or more recipients. It’s attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
said: 554 5.7.1 Service unavailable; Client host [184.108.40.206] blocked
using bl.spamcop.net; Blocked – see
http://www.spamcop.net/bl.shtml?220.127.116.11 (in reply to RCPT TO command)
What this informative yet unhelpfull error message says is that 18.104.22.168 which resolves to smtp-out31.otenet.gr is blacklisted by spamcop. What you can do is tell spamcop that this is not a smap through their page but even if you do you still have to wait for them to approve etc which in best case takes some hours.
So the alternative is to use directly one of the smtp servers of Otenet and not going through the mailgate.
To do so you can use any of these 4 ips making sure that they are not blacklisted.
22.214.171.124 smtp-out31.otenet.gr 126.96.36.199 smtp-out32.otenet.gr 188.8.131.52 smtp-out33.otenet.gr 184.108.40.206 smtp-out34.otenet.gr
You can check their blacklist status here http://mxtoolbox.com/blacklists.aspx .