Updated some stations, so the version enabling mails are more expensive. Because those stations don't use the old level system anymore, they can't be overbuildt, stating that the station needs a higher level.
I suggest checking capacity instead of level.
While not using a level in calculations, it could still set a level in the makefile.
I thought you could overwrite higher level buildings with lower ones while holding down ctrl?
Changed this with r 7092. It makes much more sense to compare capacity: for new ones this is imho the intended way to do, for old ones capacity is deduced from level anyway.
ctrl+click still forces the station to be built.