User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:18.104.22.168) Gecko/2008070208 Firefox/3.0.1 Build Identifier: 5 e-mail sent last week haven't arrived. One I dispatched hasn't been received. Reproducible: Sometimes Steps to Reproduce: 1. 2. 3.
Sorry we need a lot more information than that. Are you sure the recipients junk filters didn't eat them? Problems with your SMTP server? An smtp log might help in case you can reproduce - https://wiki.mozilla.org/MailNews:Logging
No response in roughly two weeks, so marking this one as incomplete.