[A1kmm]

I self-host my email, and have not really had problems delivering normal quantities of personal email (except a bit of pain for Microsoft to accept mail in the first place, but it can be sorted quickly) - as long as you do DMARC / DKIM / SPF.

I've never heard of t-online before or tried to send an email there to my knowledge... if one provider I've never heard of would refuse to accept my mail if I ever sent something to them, that's more of a them problem than a me problem - but it certainly isn't the norm for other providers.