It is possible as I demonstrate below but it is not the recommended way to get a unique incrementing serial number.  What you want is an IDENTITY column.

An Identity column is automatically allocated a number by SQL Server when a row is added.  The numbers generally increade by 1 each row but you can occationally get a gap in the numbering if a transaction is rolled back. 

Create your table with

CREATE TABLE MyTab

( ID integer identity not null primary key,

 Source varchar(??) etc..

)

The column doesn't have to be a primary key but I presume you want it to be in this case.

You can also mark the column as an identity column via enterprise namager table designed (I can't remember where).

When you insert the data into the table, you should miss out the identify column from the statement. e.g.

insert into t_postCard(Source, toName, toEmail, fromName, fromEmail, message, readRecept, flash, dateSent, pass) values('scccx', 'bxzcv', 'msdf', 'ssdf', 'sdfam', 'csdfsad', 0, 1, '01/01/2004', '2664')

The full details about Identity columns can be seen in SQL Books online.

The SQL for not using the Identity column is below but it will not perform so well.

insert into t_postCard(ID, Source, toName, toEmail, fromName, fromEmail, message, readRecept, flash, dateSent, pass)

Select  (select ISNULL(MAX(ID)) + 1 from t_postCard),

 'scccx', 'bxzcv', 'msdf', 'ssdf', 'sdfam', 'csdfsad', 0, 1, '01/01/2004', '2664'