I thought that I should let you know that if you're using your emails for business, you shouldn't really be using free web hosting of any kind for such thing as anything could happen, same would apply for web hosting in general, like there isn't an uptime guarantee so you could potentially lose a lot more customers than a service with such guarantee if there is downtime. There can be worse such as an "accidental" suspension that could end up with your site being suspended for good. Again, this can happen with ANY free web host.
I'm not saying that they're bad, but what I am saying is that a paid company using a free service could end up with a ship wreak, depending on how reliant the service would be for the company.
Paid would be more reliable, that's if your company does involve money being paid to the people who are involved or something along those lines.
X10Hosting (free) for instance is mainly for websites, the email part of it is there pretty much as an extra (cPanel is probably the reason) so there shouldn't really be a great reliance on that, at least that's what I believe.
I'm just assuming some parts here, so feel free to correct.
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Anyway, emails (incoming and outcoming) can usually become blocked because of abuse (e.g. spam, phishing), though in some cases, this may not be the issue.
Replying to
@Dead-i with the information that is being asked of would most likely help resolve the issue you're having.
Hopefully, this post would help, in a way.
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Nothing was harmed during the typing of this post, apart from my thumbs.