If your mail server work on old outdated TLS V1 you can allow the Splynx server to work with this server by the following config:
For Ubuntu 20.04 (from Splynx 3.1) you should add to file /etc/ssl/openssl.cnf:
after line: oid_section = new_oids
openssl_conf = default_conf
[ default_conf ]
ssl_conf = ssl_sect
[ ssl_sect ]
system_default = system_default_sect
[ system_default_sect ]
MinProtocol = TLSv1
CipherString=DEFAULT:@SECLEVEL=1
After that, php7.4-fpm should be restarted by:
service php7.4-fpm restart
Also you can try different IMAP ports: 143, 993