Move addresses from To to Bcc?

Jethro R Binks jethro.binks at
Thu Apr 22 09:14:07 IST 2010

On Thu, 22 Apr 2010, Pete Russell wrote:

> Its possible this is an MTA question and not an MS question?

I'd say it's an MTA issue really.

> Every day we see staff send email to customers and students in massive 
> lists. Everyone the message is addressed to is listed in the To: field. 
> Would it make sense to have a test for more than x entries in the To: 
> field so move all of the addresses to Bcc: ?
> If this did make sense would it be very difficult?

Here's what I say to my users when overuse of To:/Cc: occurs:


It is poor practice to send a message to a large number of recipients 
listed in the To: or Cc: field, as you have done with a recent message.

As well as damaging privacy by exposing everyone's email address to 
everyone else, it is difficult to read, and can cause problems with some 
mail clients, for example, they fail to complete downloading the message 
and keep trying to do so over and over again.

If you wish to send out a message to a large number of people, then a 
better way to do so would be to address the message to yourself or some 
other primary recipient in the To: header, and place all the other 
recipients in the Bcc: (blind carbon copy) header.  That way each 
recipient doesn't get to see the full list of recipients in headers as 
received, and it prevents the related problems.  You may need to alter the 
configuration of your mail client to make the Bcc: header available for 
completion: contact the Helpdesk for advice.

Can you please ensure that you use this method in future.  We could 
enforce this at the mail servers, but we would prefer people to do so 

If mailing a specific large group is something you are likely to do 
regularly, then the best option would be to request the establishment of a 
properly managed mailing list.

It doesn't really make sense to "move" recipients from the To: header to 
the Bcc: header.  To the email system, they are all just recipients.  
What goes in those headers is cosmetic only, and need bear no relation to 
the actual messsage recipients (but ordinary mail clients usually "do the 
right thing" behind the scenes).

In your MTA, you could refuse to accept messages where the To:/Cc: headers 
are "too large", for some value.  Alternatively you could accept the 
message and then just wipe the contents of the To:/Cc: headers (or replace 
with something syntactically valid like:

  To: many recipients:;

However, generally I would advise against automatic tinkering with message 
contents.  Choose carrots rather than sticks: after user education, maybe 
penalise messages that don't conform (maybe you could hold them on the 
queue for manual evaluation and then give the sender some more education, 
before releasing them.  Repeated delays will annoy them, but they will 
learn how to avoid them eventually.  Make the delays longer for repeated 
offenders!).  If the carrot doesn't work over time, use your stick and 
refuse to accept them!  But what you can get away with depends on your 
userbase, and what you can technically do depends on your MTA.


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Jethro R Binks
Computing Officer, IT Services, University Of Strathclyde, Glasgow, UK

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