Emails to several recipients not being received

[#1]

I have an email address with my own .co.uk domain, which I have had for many years. Recently a number of people have reported not receiving emails from me, despite multiple attempts. They do not see the emails in their spam folders and I never get any notification that the emails have not been delivered.
I know that my address is sometimes used on spoofed emails from spammers. However, that's been going on for over 10 years and hasn't caused me too much of a problem so far. I found a tool that checks an email against numerous lists of spam addresses, and it is listed on a few, but not on the majority. I really don't want to change my email address as it's my business contact email.

Could this be a problem with plus.net's mail servers?

Can anyone suggest ideas to resolve this?

[#2]

@ChrisDancer  is your domain hosted by PlusNet ? and do you always send email via the plusnet server ( relay.plus.net ) ?

[#3]

Good questions. No, my domain is hosted by accessdomain.com and I use their webmail when I'm away from my office. When I'm at the office I use an email client (Pegasus) which uses the plusnet server. It doesn't make any difference which of those I send email through, it still gets lost (I think, but I might try a test).

[#4]

@ChrisDancer  I'm glad I asked the question then, rather than assuming it was hosted at Plusnet.

I'm suspecting that it may be a lack of spf record for your domain. Some mail servers, particularly google and microsoft are getting very picky these days about spf & dmarc , or the lack of it!. The lack of spf, technically, shouldnt cause rejection of email as such, but what appears to happen is that lack of spf causes MUCH more agressive spam detection on emails.  

You can check if your domain has spf records using mxtoolbox https://mxtoolbox.com/ . Put in your domain name and do the basic lookup, then use the drop down menu to select SPF record lookup.

If you don't have one then you'll need to use your hosting providers domain control system to add one which allows sending email from their mail server and the Plusnet ones.

The Plusnet ones can be added be including the following in the spf record

include:_spf-internal.plus.net include:_spf-internal2.plus.net

[#5]

Thank you for that. SPF and DMARC are both showing "DNS record not found", which I assume means my domain doesn't have them. Sounds like that could be the problem then.

[#6]

Lack of an spf record is certainly not going to help things. I'd start with getting that configured and see how it goes. I'm afraid I dont know what domain configuration system your hosting provider has but I'm sure they ought to be able to tell you how to create one. Armed with that info I can help wit the actual spf record contents if necessary