Sending a group email without being rejected?

I use my Yahoo account to send email invitations to a monthly community meeting. I have no more than 35 addresses in my groups. I had to type in the crooked letters & numbers each time before sending, and still was rejected 50% of the time. I had to resend up to 3 times to get it sent. I also have to type in the letters & numbers when I send a single email. Why is this? What could I do to assure that my emails go through the 1st time? How many addresses are allowed in a group email? Thank you.