So far as I know, older versions of Lotus Notes do not process <br/> tags (xhtml self closing), so you need to use old style <br> (non-self closing).
If this is the case, there's a problem with the RT editor in the CampaignMonitor template/content editor because it forces br tags to the self-closing type.
I have seen this happen, but maybe the versions of Notes affected account for such a small percentage? Is this an issue worth fixing? It might also be safer for Outlook with its appalling HTML renderer.
I wouldn't lose any sleep over it.
Those older versions (6.5 & 7) had limited CSS support, and tend to break most complex nested table designs.
It's the equivalent of designing an application today to be compatible with Windows 95.