Why did mx1.sprucedesk.com connect to my mail server?

Our servers check whether an email address exists. They never send you a message.

Our projects check that a contact's email address is real before it is used, so messages don't bounce. The servers mx1, mx2 and mx3.sprucedesk.com do this with a standard SMTP conversation:

1. EHLO mx1.sprucedesk.com
2. MAIL FROM:<check@sprucedesk.com>
3. RCPT TO:<the address>, plus one random address to see if the domain accepts everything
4. QUIT, with no DATA, so nothing is ever delivered

How much traffic?

Very little. Each server makes at most a few dozen checks per hour to any one mail provider, and only one connection to a domain at a time. A temporary error (4xx) means we back off and try later.

Don't want this?

See Opting your domain out. It takes one email.