How can search to find especial text from body of email such " best regards"

Depends if you use outlook or webmail.
For Outlook you can use this code to read from the inbox:

Outlook.Application outlook = new Outlook.ApplicationClass();
Outlook.NameSpace ns = outlook.GetNamespace("Mapi");
Outlook.Recipient oRecip = ns.CreateRecipient("INBOX");
oRecip.Resolve();
if (oRecip.Resolved)
{
                    Outlook.MAPIFolder oInbox = ns.GetSharedDefaultFolder(oRecip,                 Outlook.OlDefaultFolders.olFolderInbox);
                    
                    if (oInbox.Items.Count > 0 && oInbox.Items.Count != null)
                    {
                        for (int i = 1; i <= oInbox.Items.Count; i++)
                        {
                           
                            if (oInbox.Items[i] is Outlook.MailItem)
                            {
                                Outlook.MailItem mailItem = oInbox.Items[i] as Outlook.MailItem;
                                ProcessMailItem(mailItem);
                                mailItem.UnRead = false;
                            }
                            

                        }
                    }
                    if (strRTFMessage != null)
                    {
                        ///
                    }
                    else
                    {
                        strRTFMessage = "<no emails="" available="">";
                        ////
                    }
                }
</no>

You can then save that email as a normal Textfile and do a search line by line using streamreader for example

Hope that helps