[Exim] smtp transport process returned non-zero status 0x010…

Lately I've been gettting a lot of frozen messages in my queue. When I look
at the log 'exim -Mvl <message-id>' then I see:

R=lookuphost T=remote_smtp defer (-1): smtp transport process returned
non-zero status 0x0100: exit code 1 *** Frozen

But then when I try to deliver it manually 'exim -M <message-id>' it gets
delivered.

Anyone know whats going on?

Elliot