An intermittent problem with SBS 2011 / Exchange 2010 / Mac laptop trying to send using mac mail

1 user in a domain with SBS 2011 has a mac laptop running mac mail.  All others run outlook on windows PCs and don't have any problems like this one user.

He sent me this note today:

I’m having a problem with outgoing mail. Most times when  I am trying to forward, or reply to mail in which there is an attachment, it doesn’t go out. it goes into an “outgoing mail” box. Then after 2 - 4 minutes I get a dialog box saying that “Couldn’t be sent using selected server. Try again using selected server?” The server in question is Exchange. When I click “yes”  it goes out right away most of the time. Sometimes I get frustrated, like I did today, and try to send it via Mac Mail instead of Exchange, before the dialog box comes up.  Any idea what’s causing this?

When he says try to send via mac mail, I think he's talking about his personal .me / apple email account which is also set up in the mail program.

This is when he's outside the office.  I know there's the issue with emails coming into iphones (and macs?) with attachments. If sent from outlook, the user can't see the attachment from the phone or mac? Some incompatibility with outlook vs. apple products?

Is this intermittent problem something like that - mac OSx doesn't connect reliably to Microsoft Exchange?

Or how would you go about troubleshooting this? No one else when out of the office has problems getting back in through the firewall to send mail via the in house exchange server in SBS 2011.

thanks!