The default SMTP port number for Bosch IP cameras is 25. The required SMTP port can differ depending on the service provider. From FW 5.80.0073 onwards it is possible to change the SMTP port using Alarm Task Script Language (ATSL).
Check out the section Alarm Mail of the ATSL documentation, there is detailed description of each required parameter.
Note: Firmware 6.30: it is not possible to change the SMTP port via Web Interface