Lesson Learned #263:Mail could not be sent to the recipients because of the mail server in DBMail

Several days ago, I worked on a service request that our customer got the following error message: The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using XXX. Exception Message: 1) Exception Information =================== Exception Type: Microsoft.SqlServer.Management.SqlIMail.MailFramework.Exceptions.BaseMailFrameworkException Message: Cannot send mails to mail server. (The specified string is not in the form required for an e-mail address.) Data: System.Collections.ListDictionaryInternal TargetSite: Void Send(Microsoft.SqlServer.Management.SqlIMail.MailFramework.Framework.IMessage) HelpLink: NULL Source: DatabaseMailProtocols HResult: -2146232832 StackTrace Information =================== at Microsoft.SqlServer.Management.SqlIMail.MailFramework.Smtp.SmtpMailSender.Send(IMessage msg) at Microsoft.SqlServer.Management.SqlIMail.Server.Controller.ProfileMailSender.SendMailToAccount(Account a, IMessageSender ms, OutMailItem si)