- Using mail service (MailChimp, Mad Mimi, etc.):
- They will handle tracking which addresses could not be delivered to and can provide tracking which users read the mails/clicked links via some redirector (to the extent users don't defend against this).
- You don't have to install and set up anything special.
- It ensures you don't leak the addresses in the headers.
- Using mailing list software (majordomo, mailman, ezmlm and such):
- It allows people to subscribe/unsubscribe themselves and can handle undeliverable addresses, but it does not handle tracking.
- It allows whoever you want to allow to send email to the list.
- It does not leak the addresses in the headers.
- Disadvantage is that you have to install it.
- Using outlook:
- You have to exclude failed addresses yourself, manually.
- You have to make sure you use the addresses in Bcc, not To. Writing the addresses in To is a privacy issue and huge faux pas.
- It is more complicated to get the distribution list from some other system, e.g. CRM database.
Each of the methods will easily handle the load. Depends on what extra features you want.