Many a times sending mail to the groups created gets failed or gets bounced back.This issue might be occurred because of spam filters.
So, to avoid such issues to disable spam filter for the affected group.
To disable spam filters you can follow the steps below.
1) Goto Google groups and select the affected group.
2) goto settings by selecting the gear icon on the right side.
3) then goto the settings and select moderation.
4) There you will find Spam messages option, in that select "Skip moderation queue and post to the group" option.

By selecting this messages will bypass the spam filters and will get posted to the group.