With awk:

awk -F: 'BEGIN{ORS=" "}$1=="MIME-Version"{exit}{print $2}END{print ""}' file    

With awk:

awk -F': ' 'BEGIN{ORS=" "}$1=="MIME-Version"{exit}{print $2}END{print "\n"}' file    

With awk:

awk -F: '$1=="MIME-Version"{exit}{print $2}' file
 RSMF Generator Sample Library
 1.0.0
 53
 2022-09-20T04
 2022-09-20T16
 GRP000000118
 GRP000000118_D_20220920
 False
 Native Messages
 Person One <5156242756> Person two, Person

With awk:

awk -F: 'BEGIN{ORS=" "}$1=="MIME-Version"{exit}{print $2}END{print ""}' file