Email to multiple recipients partly/mostly blocked by somebody.

Just switched from Outlook on XP to WLM on Windows 7.  Have been previously sending through website hosted by Citymax for years.  First try on WLM  at sending to 150 person list went to Outbox. No error message.. Reduced to 10 recipients at a time, worked 5 times, then got sent to Outbox, where it refused to go farther. Almost every try at dividing up my list has failed. Comcast says it's not them. CityMax claims not to be limiting outgoing. Puzzled.