lmtp Trying to unput wrong character

[Stephan]

Hello,

I am having trouble with a mail stuck in the queue, it can't get
delivered because lmtpd has some problem, it says:

lmtp: FATAL: Trying to unput wrong character

I'm not sure what to do about this. I found the error message in
prot_ungetc() which is sitting in lib/prot.c but don't know what this
actually does. If I interpret this correctly, it gets called in
pushmsg() in lmtpengine.c and the comment says that there is a carriage
return that should be put back, which apparently doesn't work.

Any hints ?

Thanks,
Stephan