Transaction failed: line too long in mail head

Hello,

On my university website, I am sending out subscription messages to quite a lot of users.

As the mail server dislikes hundreds of mail from the same IP, I am grouping them into one mail (well, they're not personalized any more, but at least, it seems to work), making a list of e-mail addresses and then using the line:

$headers['Bcc'] = $emaillist

to add the users to the mail.

It works fine with not too many addresses, but when I have too many addressees, the mail server complains:

Remote host said: 554 5.7.7 Transaction failed: line too long in mail head

Now, I wonder how I can split the BCC field into several lines, and would be very grateful for any help.

Have a nice evening / day,

Felix.