The database isn't a mail server, so it has no way to register if someone received an email.
Your two best bets would be
1) Contact your network admin and have him review the email logs to see if it is getting blocked, etc. This should be pretty easy for them to do (well it is for my network guys, but it might be dependent upon mail configuration and such).
2) Check the Reporting Service logs to see if there is a message telling that it failed for some reason. These logs are a bit different than the SQL Logs and Mail logs and often contain additional information.
Generally speak, the ssrs logs are found her: %ProgramFiles%\Microsoft SQL Server\MSSQL.x\Reporting Services\LogFiles