The article here is assumed that a SQL Database Mail profile "MailProfile1' has been created. The MailProfile1 tells Database Mail what account it should use to send emails. For how to create a Database Mail profile, you can refer to the previous post SQL Database Mail - Send Emails from SQL Server.
To delete all records that are generated before 2:37pm, January 17, 2011, run sysmail_delete_log_sp as follows.
-- Start T-SQL
USE msdb
EXEC sysmail_delete_log_sp @logged_before='2011-01-17 14:37:00'
-- End T-SQL --
Below is the comparison of log history show before and after deletion.
Read more: SQL Server Agent Team Blog