[exim] Failed to get write lock on smtp?

Hello,

I have a smtp-transport using lmtp. In the mainglog I got several like this:

Failed to get write lock for /var/spool/db/wait-mailbox_lmtp.lockfile:
timed out

What is this write lock for? mailbox_lmtp is the name of the transport.
Is this transport treated as a remote delivery so remote_max_parallel
applies?

Regards
Marten