How to build list of failed emails in foreach loop in C# -

i trying check email validation list of mails. if of emails invalid, have send list of failed mail validations false message, new c#, how can achieve this? when reading books found possible through stringbuilder, unable find syntax this, can please provide syntax this.

    public static string issmtprequestvalid(this smtpgatewayrequest smtprequest)             {                  smtprequestcontent content = smtprequest.body smtprequestcontent;                  foreach (string email in content.emailcc)                 {                     bool valid = isemail(email); /* here check in of mails invalid have return false, list of invalid mails.*/                 }              } 

thanks lot in advance.

first have create instance of stringbuilder class , append invalid emails in new lines. if want return emails in string have call .tostring() method.

public static string issmtprequestvalid(this smtpgatewayrequest smtprequest) {     stringbuilder invalidemails = new stringbuilder();     smtprequestcontent content = smtprequest.body smtprequestcontent;      content.emailcc.where(email => !isemail(email))         .foreach(n =>         {             invalidemails.appendline(n)         });     return invalidemails.tostring(); } 

or in easier-to-understand form

public static string issmtprequestvalid(this smtpgatewayrequest smtprequest) {     stringbuilder invalidemails = new stringbuilder();     smtprequestcontent content = smtprequest.body smtprequestcontent;      foreach (var email in content.emailcc)     {         if (!isemail(email))         {             invalidemails.appendline(email);         }     }      return invalidemails.tostring(); } 

moving away solutions think best solution return entire list of items. example:

public static list<string> issmtprequestvalid(this smtpgatewayrequest smtprequest) {     list<string> invalidemails = new list<string>();     smtprequestcontent content = smtprequest.body smtprequestcontent;      foreach (var email in content.emailcc)     {         if (!isemail(email))         {             invalidemails.add(email);         }     }      return invalidemails; }