Outlook 2007 not sending emails

Hello,
I am using Outlook 2007 and have several email aliases set up, all with different outgoing SMTP servers from different ISPs. Recently I have found that some of my emails are not being sent, even though it looks like they have gone - they appear in my sent items and no error messages appear.

Thios happens without any apparent reason and to multiple recipients. I have also check the trash folder of the recipients and it doesn't appear that my emails are going into any spam folders.

Presumably this is a problem with Outlook but I have no idea where to begin looking for a resolution for this issue. Any help would be much appreciated.