Click the "Opera Mail" button at the top left of Opera Mail. This is called the menu button. Then, goto "Mail accounts". Then, select the account you want to edit and click "edit". Then, on the "Servers" tab, you edit the incoming and outgoing server properties.
If the incoming server is a POP server, you want port 995 with TLS checked. If that doesn't work, you can try port 110 with TLS checked.
If the incoming server is an IMAP server, you want port 993 with TLS checked. If that doesn't work, you can try port 143 with TLS checked.
For the outgoing (SMTP) server, you want port 465 with TLS checked. If that doesn't work, you can try port 587 with TLS checked.
Authentication should be set to "Auto". For the username, if it's your email address, make sure to include the @... part. If it's just your username, make sure the @... isn't included.
Make sure the server name is correct. mail.example.com or pop.example.com or imap.example.com or smtp.example.com etc. Look up the required connection settings for your mail provider.
Agree with tcebob. Even though my download are successful the icon will alway turn red and will not return to normal without restarting Opera. I love opera browser and its multitude of useful features, especially the ability to turn individual cookies on/off, but this bug is really annoying. At least give us the ability to reset the icon or disable it from turning red. I can tell you so far I have had less than 1% failed downloads, but 100% red icon indicating a failure. BTW I am running Opera 50.0.
There were a few mysteries in opera 44.0.2510.857 so I completely removed it and reinstalled 44.0.2510.1159. Works much better all around. Youtube has an audio icon which mutes and unmutes; I thought it was a volume control so ignored it. Problem solved.