NMC2(AP9630) (AOS 7.0.4) Email send not working (TLS SMTP issue)

[aaaskew]

Have others found this issue?

An AP9630 on the latest AOS 7.0.4 cannot send emails to an SMTP server using TLS or StartTLS if the SMTP server's initial TLS negotiation is too big. My SMTP server's TLS 'Server Hello, Certificate and Server Hello Done' is 5645 bytes. The maximum size supported by the AP9630 is probably 4344 bytes, based on the TCP Window size seen in the TCP negotiation.

To spot the problem:

- The Email test fails

- A packet capture of the network traffic shows 'TCP ZeroWindow' packets from the NMC2 after the initial TLS 'Server Hello' packets from the SMTP server.

The only way of fixing this that I have found is to use another SMTP server with a smaller certificate chain.