If you are sending a large mailing to contacts from different companies, a good idea is to put their email addresses in the b.c.c. section and then send it. That way nobody will see who else is getting the email or have access to their email address. This is particularly important in a business mass email mailing.
I had a personal experience where I copied many of my friends and acquaintances on an email. One of those recipients then took the names from my list and started emailing my contacts for sales purposes. Lesson learnt, but if that had been a business email the consequences would have been greater.
Some companies have firewalls to try to control the mass emails that come into their organization. By sending mass email mailings their organization's firewall may start to recognize your email address as spam and not allow any future emails to get through.
To avoid this problem some companies contract their mass mailings out to a marketing company who will email it on your organization's behalf.