SMTP Blocking My IPv4

I cannot send emails through my comcast accounts. (Most of the time.) After days of working on this, including calls with Customer Service reps, I've come to the conclusion that Comcast's SMTP server is blocking my IPv4 address when using a third party email program (eM Client). If I use my laptop with my phone modem or through VPN everything works fine. If I plug my laptop directly into my cable modem (BTW, my ISP is Spectrum) a new IP address is assigned and everything is good. Of course, when I plug my router back into the cable modem the original IP address comes back and no outgoing mail. (Until around 8:00 PM. Then it seems to work for a little while.)

I have checked the the IMAP & SMTP settings and passwords. Also, the same behavior occurs if I use my Android phone over my Wi-Fi network. (It works fine if I force it into data mode over the phone network.)

How do I get my IP address off of the "naughty list"?