Too early to say that (I used the holidays for the transition). With your custom domains you have to make sure everything is setup properly on the DNS side (MX records, SPF, DKIM/DMARC etc.) but the proton ui has checks and makes that really easy, when correctly setup all checks are displayed "green". If you have trouble of your outgoing mails to be accepted (or land in spam), I would advice making sure all is marked green for you too.