You could use the inserted table in your trigger to get the values that were inserted in the row.

Example:

Assuming you have a table with the following columns:
 ID[int], EmailAddress [varchar(2000)], EmailCount [int]

The trigger:

CREATE TRIGGER SampleTrigger
ON MyTable
AFTER INSERT
AS

DECLARE @ID int
DECLARE @MessageCount int
DECLARE @EmailAddress varchar(2000)

SELECT
    @ID = ID,
    @EmailAddress = EmailAddress
FROM
    inserted

SELECT @MessageCount = COUNT ('C') FROM Messages WHERE EmailAddress = @EmailAddress

IF (@MessageCount > 10)
BEGIN
    -- Modify email and send message
ELSE
BEGIN
    -- Fallback do something else
END