i add phpmailer at my elgg . it woks in last 4 months. but today i find a bug at phpmailer , i don't know how to fix it , can u give me some advice?

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Redirect could not be issued due to headers already being sent. Halting execution for security. Output started in file /www/web/AA/public_html/mod/phpmailer/vendors/class.phpmailer.php at line 640. Search http://docs.elgg.org/ for more information.

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