yet you do not share either the error message or the provider to the recipient server, so someone can do more than speculate and guess. However a quick google search indicates that the vast majority of those with this issue are also sending from or to a Microsoft exchange environment. Is this the case here? Others that have images in their signature have issues with the images being identified with spam. This is particularly for those including one of those blue T or F as neither website have an official image, they use scripts to serve the image so their reach tracking folk is greater, so they are commonly web downloads used in signatures that spammers also use. This "solution" from the Mailbird email client developers/support really sums it up. https://support.getmailbird.com/hc/en-us/articles/16297820807959-Message-blocked-due-to-spam-content-in-the-message Simply substitute Thunderbird for Mailbird. Inmotion hosting also offer good advice https://www.inmotionhosting.com/support/email/stop-emails-being-labeled-spam/ The main issue is it is unlikely it is an issue with your chosen mail client, be that Thunderbird, mailbird, outlook or the Bat! so offering advice is very difficult.