its nearly definitely your smtp server, admins usually put a max size on a mail to prevent problems. its default is 20mb IIRC but they could have upped it to 25 for you. get them to up it to 30mb to see does it go through.

I know you dont want to hear it but what happens when the message is >30mb. find another way.

The actual error, "+552 5.3.4 Message size exceeds fixed maximum+", is from the smtp server. Oracle expects a 0 return code and instead get a 522 return code in the smtp server's response (to the DATA command). So Oracle throws an exception that in turn contains the error response of the smtp server.

Interesting that you did not get the error earlier - many smtp servers have the ceiling set a lot lower and you can expect this error after sending around 5MB of data.

There are ways around this. Send the attachment as multiple fragments using a compression technology like zip or rar (where it is fairly easy for the recipient to reconstruct the source from the pieces send).

There's also another problem. The 20+ MB e-mail may be accepted by the smtp server, but the POP3, IMAP (or Exchange?) account (mail drop) may not accept an e-mail of that size.

Success is also assuming that you are delivering that mail directly to the correct/target smtp server. If not, then that e-mail needs to be relayed via a number of intermediate smtp servers before reaching the target smtp server and domain.

And there is a good chance that one of these relay servers will object to the size of the e-mail and trigger a hard bounce response.

Keeping an attachment size down to at most 3 to 5 MB is IMO the better approach as that guarantees a better chance of delivery than a 20+ MB e-mail.