It sounds like you mean the user has several SMTP addresses to which they are able to receive email. However only one of them is the primary SMTP email address and trying to send “from” anything but the primary gives you the error. As far as I know there is no work around for this within a single mailbox with mutiple SMTP addresses. Only the primary will show as the “from” address.
You CAN send from both email addresses. If you edit emails using Word, Click on the arrow button next to Options and select from. If you edit emails using Outlook, the From option is in View. Just make sure the user has permissions to send as both email accounts.