I am currently using:
Windows Live Messenger 2011 (Build 15.4.3538.513)
Messenger Plus 5.10.0.748 (Released 20/9/2011)
Windows 7 x64 SP1

Problems:
When i try send a message by starting a new conversation, once i press "Enter" key to send the message then it will immediately show me "The following message could not be delivered to all recipients". (FIGURE 1)

This problems occurs for only one contact, i can send message to other remaining contacts.

However when this contact send me a message, i am able to reply him without any problems.

*This problems no longer exist when i uninstall MSGPlus.
*However when i try to install MSGPlus back, this problems occur again with the same contact person.

What i have tried:
Reinistall WLM
Reinstall MSGPlus
Clean registry with CCleaner, Tuneup Utilities 2011

And still only that contact has the problems with MSGPlus.... T_T

Thanks in advance for help~~ ^^

FIGURE 1:

hello,
i also have this same problem, i have windows live 15.4.3555.308.
is there a fix. i have messenger plus live 5.50.0.761

Uninstall Plus!. This version was never tested.

Try the previous version and see if you have any success with it: CookieRevised's reply to Messenger Plus! 5.5  for Windows Live Messenger  released

Hello,
i have tried that but it still seems to open 2 chat covos for 1 person at the same time then goes to just 1 and messages wont send but it doesnt do this when messenger plus isnt installed. any1 else have the problem and know how to fix it

Do you currently have anything else installed or did you once have something else installed related to Messenger? Stuff like SmileyCentral and the likes?
Is Messenger running in compatibility mode?

i dont have anything else installed only messenger plus, running messenger in compatibility mode still has the same problem. i noticed this problem as soon as i got windows live messenger 15.4.... its a shame really i been using messenger plus for years.

will you be making a messenger plus that works well with windows live 15.4....