Limit concurrent connections

Currently, we make two concurrent connections to a mail server during a hostname check. Making those connections sequential might help avoid aggressive anti-spam measures.

Start by locating the Go code that performs the hostname check and creates the two mail-server connections. Confirm how the concurrent connections are initiated, then verify that the check completes with those connections made sequentially and that the anti-spam-related behavior is covered by the available tests.

Written by the indexing model from the issue text.