emails not being sent when ssl enabled

I have several worddpress installations on my host. On one of them I have changed the site name to start with https. When this is done, wordpress does not send email, for example, in a “forgot your password” submission. As soon as https is disabled, email works correctly again.

How can I debug this?

I have narrowed the issue to setting the site address to start with https. On other sites on the same host where https is not used, emails do send correctly. So there is no issue with the SMTP server (postfix) or with php.

Also, if I manually change to http from https in the browser in the “lost password” page, the email does get sent.