Urls in messages are inconsistent

I'm using Simplenews and Mimemail to deliver a newsletter to ~550 members. The newsletter pulls in data from 2 different views at delivery time, along with other content entered specifically for the newsletter itself.

The problem is that a significant percentage of users are seeing broken urls in the output from those two views. For example, instead of seeing a link with url http://www.example.com/content/my-article, they're getting http://:/content/my-article. Obviously, that link won't work when clicked in an email client. The weird part is this doesn't happen to everyone; when *I* receive the newsletter, the links are (usually, but not always) intact and functional. Also, when I send the newsletter to the 'test address', the links always come out right (even if I make the test go to one of those users that always has the problem with 'normal' newsletters).

I've considered that there might be a permissions issue here...but I haven't been able to sort it out.