Spectrum provide the means for 3rd party apps via their own imap, pop and smtp server settings, but do not really have any knowledge on how anything works nor do they really care, after all they want you to use their webmail. They are not unique, it's a common issue. Usually port 465 uses Connection SEcurity: SSL/TLS and port 587 uses Connection Security: STARTTLS Try port '''587''' Connection Security: '''STARTTLS''' Exit Thunderbird allow a few moments for background processes to complate and start Thunderbird.