server is SBS2003 standard, all patched and up to date including SP2 for Exchange 2003.
smtp traffic had been flowing normally, but we now have an issue with sending messages that have attachments - of any size.
have dbl-checked the message delivery options and the smtp server options and there are no restrictions on message size.
messages without attachments get sent fine.
messages with attachments sit in the outbound queue with 'retry' displayed.
totally stumped, all suggestions gratefully receved!