import re
text = "system:microsoft flow:to_server vho:file-was-closed heur250:unknown.file,  file.png:word, 03:00,  file:123, mailto:user, cid:image003.png"
pattern = r'\bmailto:[\w.-]+|\b\d+:[\w.-]+|[\w.-]+:\d+|[\w.-]+:[\w.-]*\.(?:jpg|png)(?![\w.-])|[\w.-]*\.(?:jpg|png):[\w.-]+|([\w.-]+:[\w.-]+)'
print( [x for x in re.findall(pattern, text) if x!=''] )