Not sure why the Sys-Admin told me to use 587
Port 587 should only support a secure connection and should only support StartTLS. 587 shouldn't support a direct TLS connection. That's what port 465 is for. But, some servers support a direct TLS connection on port 587 anyway (and some servers support insecure connections on port 587 anyway). The reason that works in Thunderbird is that Thunderbird supports that configuration. The reason it doesn't work in Opera is because Opera doesn't support that configuration.
The sys-admin probably doesn't know that Opera doesn't support a direct TLS connection on 587. But, the admin should have suggested 465 as an alternative. As for whether the admin knows that 587 shouldn't support a direct TLS connection, he might know and just doesn't care as it works fine with most mail clients and a lot of servers do it. Then again, he might not know and can fix things. But, that'd cause some users to have to switch to 465, which may be undesired, especially if it's only to deal with Opera's strictness. Suggesting port 465 as one of the options should be good enough.