Can someone help us with this error message?! Thank you very much.

[FormatException: The specified string is not in the form required for an e-mail address.]
System.Net.Mime.MailBnfHelper.ReadMailAddress(String data, Int32& offset, String& displayName) +1296
System.Net.Mail.MailAddress.ParseValue(String address) +308
System.Net.Mail.MailAddress..ctor(String address, String displayName, Encoding displayNameEncoding) +120
YAF.Classes.Core.YafSendNotification.ToWatchingUsers(Int32 newMessageId) +1138
YAF.Pages.postmessage.PostReply_Click(Object sender, EventArgs e) +1600
YAF.Controls.ThemeButton.OnClick(EventArgs e) +90
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +29
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2981

No one can help to a man with an unknown yaf version.
You possibly have an error in mail.config.

Agreed - version information helps!

Either you have an issue in your mail.config or the account attempting to be notified does not have a valid email address associated with it.

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