We have a batch program that incorporates JavaMail 1.2 that sends emails. In our development environment, we haven't got the chance to encounter the above mentioned exception. But in the client's environment, they had experienced this a lot of times with the following error trace: javax.mail.MessagingException: 550 Requested action not taken: NUL characters are not allowed. at com.sun.mail.smtp.SMTPTransport.issueCommand (SMTPTransport.java: 879) at com.sun.mail.smtp.SMTPTransport.finishData (SMTPTransport.java: 820) at com.sun.mail.smtp.SMTPTransport.sendMessage (SMTPTransport.java: 322) ... I'm not sure if this is connected to my problem, http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4697158. But trying JavaMail 1.4.2, I see that the content transfer encoding of the email is still 7bit, so I'm not sure if using JavaMail 1.4.2 could solve the problem. Please take note that I could only do testing in our development environment that hasn't been able to replicate this. With the above exception, how would i know if this is from the sender or the receiver side? What debugging steps could you suggest? EDIT: Here is a DEBUG of the actual sending (masked some information): DEBUG: not loading system providers in &lt;java.home&gt;</a>/lib DEBUG: not loading optional custom providers file: /META-INF/javamail.providers DEBUG: successfully loaded default providers DEBUG: Tables of loaded providers DEBUG: Providers Listed By Class Name: {com.sun.mail.smtp.SMTPTransport=javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc], com.sun.mail.imap.IMAPStore=javax.mail.Provider[STORE,imap,com.sun.mail.imap.IMAPStore,Sun Microsystems, Inc], com.sun.mail.pop3.POP3Store=javax.mail.Provider[STORE,pop3,com.sun.mail.pop3.POP3Store,Sun Microsystems, Inc]} DEBUG: Providers Listed By Protocol: {imap=javax.mail.Provider[STORE,imap,com.sun.mail.imap.IMAPStore,Sun Microsystems, Inc], pop3=javax.mail.Provider[STORE,pop3,com.sun.mail.pop3.POP3Store,Sun Microsystems, Inc], smtp=javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc]} DEBUG: not loading optional address map file: /META-INF/javamail.address.map DEBUG: getProvider() returning javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc] DEBUG SMTP: useEhlo true, useAuth false DEBUG: SMTPTransport trying to connect to host "nnn.nnn.n.nnn", port nn DEBUG SMTP RCVD: 220 xxxx.xxxxxxxxxxx.xxx SMTP; Mon, 23 Mar 2009 15:18:57 +0800 DEBUG: SMTPTransport connected to host "nnn.nnn.n.nnn", port: nn DEBUG SMTP SENT: EHLO xxxxxxxxx DEBUG SMTP RCVD: 250 xxxx.xxxxxxxxxxx.xxx Hello DEBUG SMTP: use8bit false DEBUG SMTP SENT: MAIL FROM:<a href="newmsg.cgi?mbx=Main&[email protected]">&lt;[email protected]&gt;</a> DEBUG SMTP RCVD: 250 <a href="newmsg.cgi?mbx=Main&[email protected]">&lt;[email protected]&gt;</a>... Sender ok DEBUG SMTP SENT: RCPT TO:&lt;[email protected]&gt; DEBUG SMTP RCVD: 250 &lt;[email protected]&gt;... Recipient ok Verified Addresses &nbsp;&nbsp;[email protected] DEBUG SMTP SENT: DATA DEBUG SMTP RCVD: 354 Enter mail, end with "." on a line by itself DEBUG SMTP SENT: . DEBUG SMTP RCVD: 550 Requested action not taken: NUL characters are not allowed.