Can't send SMTP to all emails

Hi, I can send email with my asp.net 2.0 to gmail and other domains with my local SMTP server, but if I try and send it to a cellphone like 5551234567@tmomail.net, the mail just gets stuck in the SMTP queue.  I can send a text message via email to that same email address with normal email.  For example, I can send an email through gmail or my company's server to the tmomail.net email, but I can't from my local SMTP server.  I can send email with my local SMTP server to other Internet and Intranet emails, just not the phone.  My local SMTP is working, security is for localhost only, just can't send to this address.

 Thanks for any help.
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