Must issue a STARTTLS command first

Regulary I get messages like (mailaddress changed against spambots):
Could not verify email address "....@huisvoorbeeldcultuur.nl" at host "huisvoorbeeldcultuur.nl.":

Results: Connection: 250 fallback.axc.nl

From address: 530 5.7.0 Must issue a STARTTLS command first

To address: 530 5.7.0 Must issue a STARTTLS command first

What code must be issued for sending a STARTTLS command?