Could the fact that my husband is also using Outlook at the same time cause this?
If you are using POP3 and are sharing the mailbox, yes, it could be his fault.
But I'm not sure the symptoms fit.
When an email client is connecting to a mailbox using POP3, it puts a lock on the mailbox so other mail clients can't access it. At least in the case of Outlook, it brings up the login screen. It should not prevent Outlook from sending messages with 3 or more recipients - it would only temporarily cause problems.
You can test it by shutting down his computer.
If you can send mail, then it is his fault.
What you can do to reduce the effects: set outlook to check mail no less than every 7 and 11 minutes (7 on one, 11 on the other). Higher is better, use odd numbers. (Ctrl+Alt+S to open the send and receive dialog.)
FWIW, if you have outlook checking mail more frequently than 5 minutes, increase it to at least 5 minutes and see if it helps.