1- Check in your host file (C:\Windows\System32\drivers\etc) and see if following entry exist or not:

   127.0.0.1 YourSiteName

  If not, adding the above should solve the problem.

2- Check if "Everyone" has read access to news letter.

Hope above helps.

I am having same issues as well and I have checked both host file and access premissions, they are correct, is there any thing else that we have to do?

Strange thing is,it was working fine till yesterday

Same problem over here. My solution was decompiling the assembly EPiServer.Mail.Core.Web and create my own class CmsContent source and alter GetMailBody().

Replacing decompiled code:

if (typeof(Global).GetProperty("UrlRewriteProvider", BindingFlags.Static | BindingFlags.Public) != null)

{
urlRewriteProvider = (UrlRewriteProvider)typeof(Global).InvokeMember("UrlRewriteProvider", BindingFlags.Static | BindingFlags.Public | BindingFlags.GetProperty, null, null, null);
}
else
{
urlRewriteProvider = (UrlRewriteProvider)typeof(Global).GetField("UrlRewriteProvider", BindingFlags.Static | BindingFlags.Public).GetValue(null);
}

By:

urlRewriteProvider = EPiServer.Global.UrlRewriteProvider;

The method GetMailBody contains a nice try catch with no error handling. So nice error and no logging.

For me it started to work again next day, I have no idea what fixed it and what caused it at the first place