Professional services firms grow on reputation, so the first filter is not price. It is whether the agency can sell without sounding like a vendor.
Referrals still dominate, outbound extends them. Hinge's High Growth Study 2025, covering 770 firms, found the fastest-growing firms spend roughly twice as much on marketing as their peers, prioritise educational content and LinkedIn and outsource specialist marketing more often. Hinge also notes that referrals remain one of the most influential factors in how professional services are bought. Outbound that leads with expertise fits that picture. Outbound that leads with a discount does not.
Expect a slower cycle than software. CIENCE's own benchmarks for the segment put the average deal near $50,000 with a ten to twenty week cycle and about 6% of meetings closing. Judge an agency on the quality of the conversations it books, not on how many it books in month one.
Check the rules before anyone sends anything. Solicitation rules for lawyers and accountants vary by jurisdiction and by bar or institute. Ask how the agency handles opt-outs, what it will and will not say on your behalf and who approves copy. An agency that cannot answer has never worked with a regulated firm.
Separate the sending infrastructure from the firm's domain. Cold email should never go from the domain your clients email. Ask every agency below how it sets that up.
Ask for a named client in your practice area. Several agencies on this list have a professional services page and no named client on it. That is a page, not a practice.