Many small wedding businesses start out by using outlook or a similar email box to send out their emails. However the larger their email list gets, the more emails get blocked from being sent as the email servers are not set up to handle that large of an influx of email and they will automatically decide that your email is spam.
I remember the first time I had an event where I needed to email 50+ people. I tried to send one single email to the entire list and every email got blocked. So I found myself sending out 5 emails at a time. Believe me, by the time I was done I was extremely frustrated with my email server.
Check out this video by Debby Dowling of WeddingProNewsletter that explains email distribution services and why you need them.