SQL Developer

I have an email programme built using PL/SQL. It fails when the email is over 6K bytes long. It fails when the line "utl_smtp.close_data(v_connection);" is executed.
The following message/s returned :- "SMTP Transient error: 501Attribute too long" I also receive the message "SMTP Transient error: 421 Service not available"

Both of these error messages report against a valid emal address and don't occur if the email message is below 5K.
The mail server will accept emails up to 10MB

From the point of construction:
1) I collect the necessary data
2) I build 'The Email' (this is a CLOB)
3) I connect to the SMTP servicer
4) Open the connection for data
5) Then in a loop, write the data
6) exit the loop when all data written
7) At the instruction "utl_smtp.close_data(v_connection);" the it all turns to tears. Giving me "SMTP Transient error: 501Attribute too long" and sometimes "SMTP Transient error: 421 Service not available"

The programme works fine on email being only a few Kb in size.

Can anybody suggest what the problem may be and /or a method of resolving it aside from spliting the email into smaller secting then sending each section.