@deleriousx
From the FAQ:
Some hosting providers place a restriction on the maximum number of recipients in any one email message. Some hosts simply block all emails on certain low-cost hosting plans.
Subscribe2 provides a facility to work around a restriction of the maximum number of recipients per email by sending batches of emails. To enable this feature, go to Subscribe2->Settings and locate the setting to restrict the number of recipients per email. If this is set to 30 then each outgoing email notification will only contain addresses for 30 recipients.
Reminder: Because Subscribe2 places all recipients in BCC fields, and places the blog admin in the TO field, the blog admin will receive one email per batched delivery. So if you have 90 subscribers, the blog admin should receive three post notification emails, one for each set of 30 BCC recipients.
With a setting of 1 – each of your subscribers gets an individual email.