Errors in sending and receiving emails with ProtonMail are often common, and these issues can’t exactly be attributed to you alone. First, determine whether or not there is an outage throughout the lowdown of all ProtonMail services by checking the status page of pages or other communal zones where messages and updates are often found.
In the case of sending issues, make sure that you do not go beyond the already defined daily sending limits imposed by ProtonMail to prevent spam; these are particularly applicable for the free accounts. Also cross-check whether the addresses you are sending to are correct or if there are some spelling mistakes.
Receiving problems can be the result of spam filters being set incorrectly, or the user blocking the sender. You can look for errors in the settings, and if emails are sent to an incorrect address, the mails may well be found in your spam folder.
Apart from that, other places such as the server may also be the cause as to why you may be unable to send or receive any messages. So log in once more and check to see if your internet is working properly. If there is a mobile application for ProtonMail, try to leave it for some time and connect using only mobile internet in order to find out whether the problem concerns the network.
If any problems concerning the sending or receiving of emails persist, the only person who is really capable of assisting you would be a support staff from ProtonMail. They should offer appropriate support as well as possible solutions with respect to your account as well as technical support problems.

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More Info: https://www.resetprotonpassword.com/proton-mail-password-recovery-problems/