As of ISPConfig 3.0.4, the text of the welcome email message that is sent to new email accounts is stored in the folder /usr/local/ispconfig/server/conf/mail/.
The message has this format:
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From: ISPConfig <postmaster@localhost.localdomain>
Subject: Welcome to your new email account.
Welcome to your new email account. Your webmaster.
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The first line starts with "From: " followed by the sender email address. The second line starts with "Subject: " followed by the email subject. The third line should be empty. All following lines are the email text.
The email text files are named "welcome_email_<language>.txt" where <language> has to be replaced with the system language like "en" for English or "de" for German language, e.g. "welcome_email_en.txt" for the english welcome email.
The customized language file should be stored in the folder /usr/local/ispconfig/server/conf-custom/mail/ so that it does not get overwritten by ISPConfig updates.