Heirloom problem with html

Hello everyone.
I've got very intrusting situation with Slackware 15.
I'm using this OS with few dell servers.
After some practise with 14.2 I started using new, official version.
I'm very happy for this distribution, because it's stability and quite simple configs.
About 2 mont's ago I started using Heirloom mailx for sending emails with html code.
I was very confused when after rebooting server stopped sending messages.
My observations:

1. No problems sending text messages, at all.
2. All messages are sending using crone automatics.
3. After restarting crone, sending html messages is possible.
4. Very strange, all machines same situation.

Maybe some advice?
I'm using mailx with M parameter, configured as MTA.
dead.letter seems ok, whole email looks ok, no errors.