Swiscom Bluewin already has connection problems on SMTP now and then.

Context: Swisscom is the biggest telecom provider in Switzerland. If you are customer of them, you get a free e-mail address on their ad-ridden and cumbersome Bluewin. They do provide you (somewhat hidden in support documents) their IMAP/SMTP addresses. I’m on a paid provider now, but my dad isn’t and has about a hundred friends and accounts, for professional use too.