SMTP Email

Just a thought - could the server still be using SSL? The email library seems to try to enable TLS (line 1880 in system/email/Email.php).
Just try changing that to another crypto type, for example STREAM_CRYPTO_METHOD_ANY_CLIENT, see https://www.php.net/manual/en/function.s...crypto.php

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Or... investigating further: STREAM_CRYPTO_METHOD_TLS_CLIENT = TLS 1.0 only since PHP 5.6.7 it seems.
If the server is modern, then TLS 1.0 is probably not supported and 1.1 or 1.2 is required.

I just queried the server with the SSL Labs tool. You might be on to something there, the server supports:

• TLS 1.3
  
• TLS 1.2
  

and no other versions of TLS.

It also supports the following:

# TLS 1.3 (server has no preference)
TLS_AES_128_GCM_SHA256 (0x1301)  ECDH x25519 (eq. 3072 bits RSA)  FS 128
TLS_AES_256_GCM_SHA384 (0x1302)  ECDH x25519 (eq. 3072 bits RSA)  FS 256
TLS_CHACHA20_POLY1305_SHA256 (0x1303)  ECDH x25519 (eq. 3072 bits RSA)  FS 256

# TLS 1.2 (suites in server-preferred order)
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (0xc02f)  ECDH x25519 (eq. 3072 bits RSA)  FS 128
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013)  ECDH x25519 (eq. 3072 bits RSA)  FS  WEAK 128
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (0xc014)  ECDH x25519 (eq. 3072 bits RSA)  FS  WEAK 256


Which is sort of interesting, but probably irrelevant.

According to the PHP manual stream_socket_enable_crypto should support TSL1.2 Client, which should be OK for this.
I am running PHP 7.3, but could bump to 7.4 without any problems on the code side.