You should do something with the problem of no arriving e-mails to some providers. It's not a good solution to say users to not use their accounts. If you have a problem one day may appear, that more accounts will stop to work with the forum. For example, if your e-mails do not follow the RCF, a server may stop to receive them after a software update or if you send e-mails directly to a receiver's server, it may stop respond due to anti-spam tweaks.