Port 2525 which by the way mirrors Port 587 is sometimes recommended by Email Clients as an alternative to Port 25 in the event Port 25 is blocked (which is not that uncommon due to inherent vulnerabilities). Fact is Port 2525 is not endorsed by either the IETF or IANA so I would personally recommend using Port 587 which does get scanned by Avast as we know. Ports 25 and the alternate Port 2525 are generally only recommended when either 587 and/or 465 cannot be used for SMTP protocol depending upon the encryption protocol used. Why Port 2525 is recommended by your Email Client and/or why Avast does not scan Port 2525 I have no idea, but Port 587 is widely recommended for SMTP protocol so I'd simply go with this SMTP protocol Port that works.
A little background; back around 1981 Port 25 was established as the default transmission channel for Internet email as a proposal by USC to the IETF and is still used today as a means of transmitting between mail servers. Around 1999 the RFC proposed a concept that involved a split message transmission relay that was determined by the RFC to occur over Port 587 to ensure a higher level of security that in turn would not interfere with relay message traffic over Port 25. Later the IANA registered Port 465 for SMTPS traffic in order to provide a Port for the SMTP protocol using SSL and later the increasingly secure TLS encryption. Currently SSL/TLS is generally recommended for Port 465 on the other hand Port 587 is recommended for the more recent protocol command STARTTLS. The STARTTLS command protocol allows SMTP servers to determine whether or not destination Servers support TLS encryption and if they do then the sending Server will upgrade the connection to the STARTTLS SMTP command protocol.
Bottom line: Port 25 and the more recent alternative 2525 Port does continue to primarily be used for SMTP relaying (FYI; SMTP is the transmittal relaying of Email between Servers). However that said, currently in most cases SMTP clients should not use these Ports in my opinion and the fact is Port 25 is very often blocked by ISP and cloud hosting providers (which is why we have the alternate Port 2525) in order to try and curb SPAM that can occur between compromised Servers. So unless you happen to be managing a Mail Server it is generally highly recommend to avoid using Port 25 (or in my view 2525 as well). Port 587 is the default mail submission port and should always be the Port used to route Email by a proper Mail Server when possible, that is unless you're explicitly blocked by your upstream network or hosting provider. Port 587, coupled with TLS encryption, will ensure that email is submitted securely and follows IETF guidelines. Port 465 is generally used for legacy systems that are only capable of using this connection method therefore Port 465 is typically used in the event your application demands it. In regards to Port 2525 discussed here in this topic, which as I said mirrors Port 587, yes it does support TLS encryption and is generally not blocked to the extent that Port 25 tends to be but as I said before is not currently endorsed by either the IETF or IANA. Because 2525 happens to be a non-traditionally high port number it is generally not blocked by ISPs and Cloud Hosting providers but nevertheless in my opinion should only be used in the event the other Ports previously mentioned happen to have connectivity issues.