Same error message. I just updated Contact Form 7 and this may be a versioning problem. I am using different Contact Form 7 instances and simpler ones with single recipients work, but the multi-selection get your error message. I have verified the alias addresses are forwarding just fine. (See code below.)
## Simple ##
Form:
<p>Your Name (required)
[text* your-name] </p>
<p>Your Email (required)
[email* your-email] </p>
<p>Subject
[text your-subject] </p>
<p>Your Message
[textarea your-message] </p>
<p>[submit "Send"]</p>
Mail:
Webmaster@Eastrose.org
[your-name] <[your-email]>
[your-subject]
## With multiselection ##
Form:
<p>Your Name (required)
[text* your-name] </p>
<p>Your Email (required)
[email* your-email] </p>
<p>Subject
[text your-subject] </p>
<p>Earliest release date: (optional):
[date date-251]
<p>Discontinue publishing after (required):
[date* date-348]
Submit to:
[select your-recipient multiple
"WebMaster@Eastrose.org"
"Petals@Eastrose.org"
"eNews@Eastrose.org"
"OrderOfService@Eastrose.org"]
<p>Your Message
[textarea your-message] </p>
<p>Attachment (optional):
[file file-645]</p>
<p>[submit "Send"]</p>
Mail:
[your-recipient]
[your-name] <[your-email]>
[your-subject]
I am not including a link to my website by intention. Thank you for the reminder, however.