I think I have the answer for you - not sure I fully understand your problem, but I have found that I cannot send email from my "primary default" address when I am not online with the ISP that provides the address (error message about not allowing relaying)
No problem when using dial-up where I live,but using Freeserve in UK, Comcast in USA, satellite connection from home, or whatever I can get when I'm somewhere else, this poses a problem.
Okay, I could use my GMail address, but I prefer not to confuse my correspondents with different addresses (and too lazy to set up the others so they recipients "see" my primary address wherever it comes from)
Due to a different problem (cr@p internet connections some places that I go) I signed up a few years ago for an "Internet Acclelerator". I found that a side-effect of this software is that my ISP doesn't "see" outgoing emails - if you inspect the headers of mails from me, they appear to come from "smtp.turbodial.net". So in addition to speeding my emails, this also cured the SMTP problem! I can now send emails from my usual email address, however I am connnected.
Do a Google on Turbodial (or turbodial.net). I think I pay $25 or $50 a year for the service, but just the side-effect of the SMTP "hosting" is worth it when I am travelling.
(the accelleration feature is good too! I still leave it running when I am on broadband!!)