I'm also having this issue on Arduino NANO 33 IoT using EMailSender library with WiFiNINA. I'm interested in finding the root cause here.
I've traced this down to the following sequence of calls (so far):
EmailSender::send-->WiFiSSLClient::connect*-->WiFiClient::connectSSL**-->WiFiClient::connected
*this fails giving the "Could not connect to mail server" after 2x successful tries)
**There is a hardcoded loop checking WiFiClient::connected() 1 time per millisecond for 10 seconds;
WiFiClient::connected() fails if socket is not available/ready
I do notice that EmailSender::send seems to create and start a WiFiSSLClient, but never stops it. Unclear if this is a problem...