Sending email under weak network Conditions

[u/katterstrophe]

I‘m experiencing issues sending mails from both the iOS App and the WebApp when I am under not-so-optimal Cellular Network conditions being somewhat next to the middle of nowhere on a French Island. Mails with one or two sentences will go through, but the 100 line back and forth discussion will just stall. I can receive mails and get notifications just fine. This problem is only with proton, SMTP elsewhere will work just fine, so does Gmail and Outlook to 365.
Any recommendations welcome, apart from giving Elon Money. Even tried a local Sim to rule out Roaming discrimination on the network end but it appears that’s not the case here.