Inbox settings sometimes play a large role in the matter. For instance, they might choose to filter out any non-business-related content (it sometimes might be the case with business inboxes). You should also avoid any illicit-sounding terms like “Male enhancement pills,” “Work from home,” “Make money instantly” because they also might alert the inbox that you’re sending spam.
Internal firewall issue?
Sometimes, especially with corporate and university filters, regular emails look suspicious and are blocked. Check the allow listing information (other providers refer to it as “whitelisting”). If that’s the case, you should alter inbox filtration settings and create an exception/whitelist sendersrv.com.
DKIM and SPF records
Basically, DKIM attaches a ‘signature’ to the message in your emails. Not literally, of course – this ‘signature’ is in the code of the message itself, which allows sending emails on behalf of a specific domain. You can learn more on DKIM/SPF records by clicking here.