First, check via webmail that this is not happening. Second, post screenshots of both accounts server settings and both account's SMTP server settings. See screenshots if not sure where they are located. Seeing those may offer some suggestions Thank you.

thanks for your suggestion, but I already checked this and both account settings are identical

OK, if you say that webmail is working properly, and that the settings are proper for both, then there are no problems. Seeing nothing, there is nothing we can offer, other than a suggestion to check filters.

See https://support.mozilla.org/en-US/questions/1438310