So I click the link in your post and look up the error and it says. <blockquote> Spectrum limits the number of concurrent connections from a sender, as well as the total number of connections allowed. Limits vary based on the reputation of the IP address. Reduce your number of connections and try again later. </blockquote> The IP address given https://ipinfo.io/75.83.239.228 is a Spectrum residential service. See https://ipinfo.io/75.83.239.228 Just a shame Spectrum do not tell us what the limits are. So I can only guess as to what might be the issue here. * Spectrum are a bunch of idiots and are refusing to accept mail submissions from their own customers. Sounds drastic, but they would not be the first US based ISP to make a complete hash out of their SMTP server by doing exactly that. * You genuinely are already connected to their server multiple times. If you have a phone turn it off, wait 10 minutes for connections to time out and try sending again. Historically iPhones tended to consume all available connections to a mail server as iPhones are popular in the USA where Spectrum is located you might have one of those problem devices. * The limit set by Spectrum is particularly low and Thunderbird 5 connections is exceeding it. GO to account settings and in server settings select advanced and change the connections limit to say 2. * Check the outgoing server (SMTP) entry in account settings and ensure that the outgoing settings include an authentication method. I have seen occasions where folk set it to none and then get this sort of error as the limit of connections when not authenticated is Zero. As I said I am guessing, as the only ones that can conclusively answer as to why spectrum is rejecting your mail is Spectrum.