Hi mgb. You'll have to be a lot more specific in what problem you encounter "stop send mail" isn't enough. Also, you've posted in the Contribs 8.X section, and last, SME7.5 is EOL for more than a year now, you should upgrade to 8.1 (or 9) ASAP.
Regarding your problem, you should take a look to either /var/log/qpsmtpd/current or /var/log/sqpsmtpd/current (depending on the use of plain SMTP, SMTP over TLS, or SMTP over SSL on your client)