After starting the service using sysmail_start_sp run the following script. This will solve the issue

SET NOCOUNT ON

Declare @ch uniqueidentifier;

Declare @message_type nvarchar(256);

Declare @message_body nvarchar(max);

Declare @ctr bigint;

Declare @ctr2 bigint;

While (1=1)

Begin

Receive @ch =...