outlook to phone text or mmms not working

Please answer a question that has never got an answer.  Is there a reason ??              
Messages from Outlook to AT&T phone    DOES  NOT WORK!    why  not?       All other accounts work .....GMAIL  YAHOO AOL  etc...
There are numerous message questions in At&T   forums   as well as AT&T direct support.   
ONCE AGAIN your department has been supplied with error messages     and even        Diagnostic  information for administrators:    Many times. 
 ------    Remote server returned '550 5.4.300 Message expired -> 450 4.2.0 
What are you going to do to resolve this matter.    TODAY!   Remote server returned '550 5.4.300 Message expired -> 450 4.2.0 <[email scrubbed]>- dudDp3DOjK2ZWdudJpECGn - Service temporarily unavailable'
Messages from outlook to AT&T phone    DOES  NOT WORK    why  not?       Simple question asked numerous times.   What is the solution??  please advise.  TODAY

Remote server returned '550 5.4.300 Message expired -> 450 4.2.0