There are no hashtags on lemmy. AFAIK, as long as you are using a mastodon instance that federates with any lemmy instance, hashtag filtering will not work to exclude unwanted content from this lemmy instance.

Different providers have different spamfilters, different rules regarding html mails, attachment file size, use of tls, policies regarding exspired certificates, and might have different log in procedures, so yes, if there are problems the question which providers are involved.