To be clear, is sender sending from Gmail and some of the recipients are also on Gmail?
In that case, Google believes she's sending spam -- probably because other recipients are clicking the Spam button. Moving hosts will only be a temporary fix, because she's likely to gain a reputation as a spammer at the new service, also.
Google's advice in cases such as this is to not use Gmail for mass emailing, but instead to use Google Groups.
3 km line on THTG: 14/1 Mb/s with Pulse8