Bulk mailing can lead to your IP being blocked or blacklisted, especially when you send these emails with phrases associated with spam emails.
You can avoid your bulk email server from getting its IP blacklisted by –
a. Sending newsletters in small batches.
b. Using segmentation option to segment your mail list.
c. Sending newsletters through more than one dedicated email SMTP server.
d. Avoiding emails that appear as spams or look sketchy