The Quickmail block is probably the way to go for a mass email to external email addresses (guessing from what you wrote your users are not actually members until now in which case the internalmail won't be of much use yet).
The only real issue is the security of the password i.e. to send a mass email informing usernames and passwords is not the best way to go.
There was a recent post in the forum along similar lines with advice from people with a lot more experience with Moodle. If I remember, using "changeme" for the password was one of the best options but still in no way secure.
Maybe the solution for the security issue is the username rather than the password. For example, if you could think of a format for the username that no one else would know (some personal data e.g. depending on what information you have you could tell them the their username is "First 2 digits of phone number + initials + birth day and month") and use changeme as the password for all of them.
If you find a solution let me know, just found ouit I have to a mass enrol in the next couple of weeks so this is good timing (although being a corporate site not such an issue as people won't mess around as kids or an open system potentially may).
Maximum studens in class
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