Invalid certificate window when sending email from shared mailbox / alias

Hello all

we are using in our environment mail system in following way:
 - we are using personal mailboxes with aliases "firstname.lastname[at]example.com"
- we are also using shared mailbox with common alias "shared [at] example.com" and we have mapped this account as "additional mailbox" through account settings in "advanced" tab
- we are using Email signing for emails with personal alias (firstname.lastname [at] example.com)
- but we also sending emails with "from:" field of shared mailbox alias

Everything is working fine, but we experiencing following issue:
- when we sending emails with shared mailbox alias, we always see "Invalid Certificate" warning window with information, that there is no existing certificate which can be used for "shared [at] example.com"
- we always need to click on "Options" tab and unclick "Sign" button for successfully sending this email.
- and this is very annoying as we are using shared alias frequently

Is there any possibility to tell to Outlook that we want to sign only mails with our personal aliases ("firstname.lastname [at] example.com") or NOT sign mails from our shared mail alias ("shared [at] example.com")?

Many thanks

ps: we are using Outlook 2010